The start line for Barbarossa would be further east, putting more pressure on the Soviets in 1941.
Which strikes me as a more than fair trade for the Soviet position having a lot more moral integrity among the Western allies.
OP explicitly puts speculation as to the reason the Kremlin does not take Hitler's deal out of bounds, which is a bit unfortunate as exactly how the war plays out depends a lot on the details of what sort of decisions are made in Moscow, by what kind of leadership. If we assume Stalin remains in power, and hews as close to his OTL decisions and mentality as is consistent with not taking the Pact deal, then overall not much is improved morally, and indeed the USSR is in an objectively worse position. We have to remember that not only the territorial status of eastern Poland (including a lot of territory annexed to Belarus outright OTL) but the three Baltic states south of the Baltic and Finland are also in play. OTL, the Pact included giving Stalin a free hand with Hitler's blessing in this region, and Stalin of course eventually occupied and annexed the three southern nations into the USSR--only to lose them when Barbarossa overran the Red Army there too. Finland of course resisted, and gave the Red Army enough of a headache that their ultimate capitulation was on harsh but bearable terms leaving the nation independent--and as things turned out, joining in Hitler's Drang nach Osten schemes.
If there is no apparent thaw in Reich-Soviet relations, and Stalin knows any move he makes in any of the four Baltic states could be the pretext Hitler is waiting for to swoop in as the ally and protector of these governments, will he keep hands off all four? And if the Germans are free to plot and scheme with these anti-Communist governments to augment their strike against the new Reich border with the USSR--presumably on the far-east flung former international border with Poland, and not farther west, will they gain such advantages, between their more easterly position and perhaps using the four Baltic nations' territory to strike immediately at pre-war Soviet borders directly, that they can break the Red Army completely and triumph in the east?
I have my doubts about the latter, even assuming that 1) the small Baltic regimes even dare to materially assist Hitler openly and 2) that the Soviet forces are in exactly the same disarray as they were OTL.
Now if there is no Ribbentrop-Molotov pact, a lot of the details of how the Soviet Union was run between the OTL date it became apparent and was announced to the Soviet peoples, and the launching of Barbarossa, would be quite different. The Soviets have every reason to expect Hitler will be attacking them sooner or later, consistent with his long stated agenda and with the Reich's policy, uncontradicted until the Germans approached the Russians in 1939 OTL and indefinitely after that here as well, to be on a crusade to the death against Bolshevism. OTL the Pact included some working agreements with the Reich, including asking the Soviet government to round up and remand various people who had taken refuge from Hitler in the USSR to Gestapo custody--and Stalin did that. Naturally all those people won't be betrayed in this fashion if relations remain frozen. I am certainly aware from some classes I took studying this era in the USSR that Soviet citizens generally loyal to the Communist regime were deeply shocked and dismayed at this sudden friendship with Hitler--all that demoralization would be sidestepped.
A more knotty question is whether the Red Army and other Soviet military forces would undergo the devastating political purges they did not long before Hitler did attack OTL. On one hand, if the old guard Stalin eliminated remains in charge and the military ranks are not in disarray from this coup, perhaps the Red Army forces would be better coordinated and respond more capably, offsetting any advantage Hitler gets from a more easterly strike. OTOH, while I don't doubt the purge mainly killed or otherwise removed mostly people who were loyal to the Soviet regime on the whole, it is entirely possible Stalin knew what he was doing regarding their uncertain support for him personally, and that while fending off the Wehrmacht more effectively on the front, its commanders might after all also carry out a coup removing Stalin from power. And while it is abundantly clear the Red Army was badly shaken up and the politically favored crop of new officers was not very competent and largely shaken out over time, the ones who did not buckle down and shake down to be more effective anyway, in favor of more pragmatically competent commanders, is it so clear that the unpurged officers would be all that more effective against Hitler's strike when push comes to shove?
Two other major considerations:
1) another aspect of the OTL Pact was that the USSR agreed to supply the Reich with various material goods. Without a Pact Hitler can't use those imports toward victory against France and the other nations of continental western Europe, nor of course then stockpile them for Barbarossa.
2) it is certainly clear that Stalin was fooled into thinking either that the Reich would never attack, or more likely that at any rate they could not attack until the conflict with all Western allies, including Britain, was settled. This false confidence that the time for open conflict was not yet at hand when it actually was surely contributed to the Red Army's initial ineffectiveness. In an ATL where no Pact is offered, Stalin would have little choice but to either plan on a preemptive strike of his own, or prepare Soviet defenses for an onslaught. OTL it seems he fell between stools, and used the farther western positions he got in Poland from the Pact to prepare for an eventual first strike of his own, which was not ready, and was badly laid out to enable an effective defense. In the ATL I think caution would prevail and rather than sending the Red Army on adventures to grab what he could get with Hitler's connivance, it would be deployed to defend the borders and probably be considerably more effective at that task that it proved to be OTL in the summer of 1941.
In that case, with the USSR's track record untarnished by the seizure of the Baltics and the attempted conquest of Finland--well, lacking the experience of poor performance in Finland is a mixed bag for everyone; OTL foreign observers downgraded their estimates of Red Army capabilities based on the black eye the Finns were giving them--without this bitter experience probably the Red Army is not quite as prepared for invasion as they think they are, but vice versa foreign observers have less reason to underestimate them. But anyway on the moral plane--whether or not the three small Baltic republics do lend themselves secretly in advance to Hitler's schemes, the Finnish state is not nearly so likely to openly aid the Reich. Certainly some factions, I would guess the large majority, of Finns are not fond of the Soviet regime, not trusting it and having reason to fear its size and greed, but a minority of Finns are sympathetic or outright Reds themselves, and while they might not be much represented in the government, they are in a position to cry foul. OtL the Winter War turned Finland into a solidly anti-Soviet nation cooperating with Hitler with a clear conscience; if there is no Winter War, Finland will I think remain neutral.
And worldwide, both committed Communists and those with some sympathy or at least tolerance of their radicalism and slavish devotion to the Moscow Party Line will be spared the whiplash of being told that all of a sudden they believed in the Third Reich and were against the French and British governments, only to turn back again to their former anti-fascism when Hitler showed his hand in 1941. Liberal regimes will be spared the spectacle of Stalin's utter cynicism in the Pact interim.
Call me crazy but I think moral integrity counts for something. I believe then that in this sort of defensive USSR scenario, any advantage the Reich gets by gaining more eastern territory and possibly being able to use the three southern Baltic republics as staging grounds as well are overall offset by a more solid defense of the USSR at its interwar borders, and that while I don't doubt the Red Army will lose considerable ground and have a steep learning curve to become more effective, they will not overall wind up worse than OTL, might wind up with the German first strike stalling farther west despite their more easterly starting point, and that when the Soviets do halt the advances and then start to push back, they will do so earlier, farther west, and come back stronger than OTL.
And this is before trying to factor in how much weaker the Reich is, at each stage of the game, without the tribute the Pact extorted from Stalin--not to mention the need to station more force on his eastern borders being unsure Stalin would not launch a preemptive strike of his own.
The OP question then shines a harsh spotlight on Hitler's motives to contradict himself on a massive scale and offer what appeared to be genuine and permanent cooperation with the USSR. OTL the Germans in general and Hitler in particular benefited overall from it, and so it is hard to figure why this deception is not attempted in the ATL too. (I have little doubt that, offered such a deal, Stalin would take it, so the ball is in Hitler's court).
