I see the logic in your argument regarding mass production of Is's but what I can say is that they are extremely expensive, it takes a while to gather the resources to produce even your first Is before even producing a second.
What I would suggest in this case (Since they are no trouble in Pop cap mode, there is no need to nerf the pop capacity) Either increase the manpower or fuel cost of the IS-2 to give disincentive to save up for an Is-2 since if one dicides to, they'll have to issue some spending cuts on reinforcing and deploying other stuff.
Or it could be done that the accuracy of IS-2's against infantry is decreased, I agree with you when you say they are too accurate against infantry, that needs to be decreased.
Although in no pop cap game mode I think this will make the game "fairer" I myself am a soviet player so. *Decrease costs of IS-2.....*