Except there are numerous ways the Nazis *could* do better, but almost none them involve new wondertech. Standardizing the motor park for one pre war. Big possible improvement. Getting locomotives that can run on the Russian rail system. Big possible improvement. Rationalizing Industrial production earlier in the war, big possible improvement. What *won't* be an improvement is shiny, finicky new wonder weapons that further overextend the already patched together Nazi economy, provide minimal combat benefit, and in this case, have been around long enough to provoke an allied response.
If the question was "An ASB causes the Wehrmacht's entire supply of 98ks to be converted to StG 45s, and replaces and upgrades German factories to support it, in 1939, how big of an impact would it have?", then yeah, that totally would lead to a moderate improvement for Germany; No one has time to respond and it comes out of the blue and Germany has the infrastructural improvements to support it. But that *isn't* the question. Whenever one nation in the interwar period demonstrated some shiny new piece of kit, everyone else at least looked into how to counter it. In order to have an StG 44 analogue in 1939, Germany would have had to have been working on it long before, and have been gradually rolling it out. In that climate, it's just another invention for everyone else to respond to, and respond to it they will. Maybe not super well, but they'll at least look into it, so that by the time the battles in France play out, they'll probably have some countermeasures.
The truth is that Germany had issues arming its army with 98k's OTL, and the StG 44 is a good deal more difficult and expensive to manufacture, as well as having a higher burden in terms of ammunition consumption and parts and maintenance. Germany was also *constantly* running extremely close to the edge of economic collapse in the prewar period, and was effectively shredding it's economy during the war, such that even minor increases in the overall burden on the economy could have catastrophic consequences.
No I mean it you really am. Upset about getting warned? No, warry? Yes. I don't do the same mistake twice, so if you want me warned or kicked you'll have to do better then that.