This is a question that has been on my mind ever since I read Spufford's Red Plenty, the tale of how the Soviet command economy tried and failed to keep the promise of abundance for all.
The first two Five-Year Plans created tremendous social and economic dislocation in the USSR--famine, sudden rural exodus, forced relocation of millions of people, etc. The country had barely adjusted when Germany invaded and laid waste to huge swathes of its agricultural and industrial heartland. Between the casualties and the material destruction, the Soviet economy was dealt a blow from which it hadn't fully recovered at the time of Stalin's death. One symbol of the cost of the war is the Palace of the Soviet: its partially assembled steel structure was hastily dismantled to feed the war machine, and afterwards all that was left of the construction site was a big, water-filled hole, that was later converted into an outdoor swimming pool. (The cathedral that originally stood on the spot has now been rebuilt).
So, my question is: if for whatever reason (say, a run-of-the-mill authoritarian regime takes over in Germany instead of the Nazis--the precise reason falls outside of the scope of this thread) WW2 was delayed for several years or preempted altogether, how would the USSR turn out economically? First in the short run under Stalin, and then in the following decades?