"I've noticed recently that SU-100 deals tremendous damage against base buildings- 3 of them can destroy it whole in mere seconds, and taking into consideration their resistance to Panzerschrecks due to upgrade it puts enemy into world of hurt."
I don't know where did you come out with this. SU 100 its acting almost like a StuG.
You probably haven't seen what a KV2, can do to poor techs. It would be OP if more than 1 KV-2 would be allowed.
Even the ISU 152 isn't that great vs buildings (static targets), probably having a little bit more damage than the SU 100. I remember it took some time to demolish a PE T1. Right now i'm not happy at all with the ISU 152, its supossed to kill the toughests cats but its not even able to hit the target, not to mention its poor reloading time. I think it should be more like the KV2 stats wise.
offtopic: su-76 destroys a puma in 2 shots, while the SU 100 in 3 shots. You better do something about the SU-76 because right now its the best AT unit and the cheapest, not to mention that you can turn in it into arty if you want.
knowing what you want is half the battle