One aspect of Stalin's reasons for ignoring the specific warnings of the actual Barbarossa invasion that, at a glance (forgive me if my glance, under some time pressure, overlooked it!) hasn't been mentioned yet is that the actual invasion was not the first time Stalin had gotten such warnings. There were, I believe, a fair number of much earlier false alarms. He also considered that after all the British would very much like for him to turn on Hitler and were scheming with all their infamous ingenuity in matters of intelligence and covert operation to persuade or trick him into doing so; he therefore attributed the latest reports, along with the earlier ones he also avoided reacting to, to such disinformation--relying of course on the aforementioned wisdom that Hitler would be insane to take on the USSR before coming to final terms with Britain.
Another consideration I've read, that shook me with its plausibility based on its arguments, is that Stalin was actually intending his own surprise attack on the Germans rather soon in the summer of '41; this is based on a Russian historian whose name I forget for the moment who has argued that his deployment of troops (and also, he claims, recently disclosed Kremlin documents which make his case conclusively) was a poor one for purposes of defense but a good one for purposes of attacking the Reich. That is, instead of developing the recently seized Polish and other Eastern European territories Stalin took at the time the Germans first attacked Poland as in-depth defense zones and retaining a strong concentration of Soviet forces behind their previously developed defensive borders on the old frontiers of the Soviet Union, which would be the proper anticipation of an invasion from the west, he pretty much abandoned the old frontiers and moved Soviet forces as far west as he could--which gave them no defensive depth but would be essential for a planned attack to the west.
I'm still not sure if I believe this as the "Stalin believed his treaty with Hitler was solid and wouldn't be violated for years" theory had been stressed by most scholarship I'd seen up that point; while earlier generations would not have seen Soviet documents now available they would surely have had enough knowledge of Stalin's force deployments to know, if these strategic arguments have weight, which way they pointed. Of course Stalin believing Hitler would not violate the treaty this early says nothing about whether Stalin would plan to violate it first!
Anyway the question in this thread is WI Stalin had known that this particular rumor of German intentions was, unlike the others, actually true and immediate.
If he really were convinced that the actual German attack was scheduled for very soon, given his front-line, aggressive deployments, the most effective defense would indeed have been to attack first, at whatever cost and despite his plans not yet being fully ripe. An alternative would have been to spike his own immediate war plans (if any) and using as much secrecy and misdirection as his regime could muster, withdraw much of his force back to the old frontiers and set about refortifying them, and redraw standing orders to the forces remaining forward to use the depth of the new conquests to buy time for the main defenders.