The IS is pretty lackluster as a Tier VII heavy- the Russian 122mm gun lacks penetration, accuracy, and RoF. The high alpha damage doesn't mean much if you can't actually penetrate (or even hit) the target, but the IS-3 is quite powerful once upgraded. It's also got a very low profile for a heavy tank. My experience with Soviet tanks in general is that they tend to be good when fully upgraded by it can be a serious chore to reach that point, unlike American or German tanks which tend to be more balanced in their upgrade paths. Of course, at Tier VII and VIII, you've got the added problem of facing Tier IX and X tanks and tank destroyers on a regular basis and against something like a T95 or E75 you're generally completely screwed regardless of how upgraded you are: I can remember times when I got my KV-3 behind an E75 only to watch my shots bounce off the back of its hull. It, of course, had no trouble penetrating my armor from any angle.