Do not exaggerate please!
We want ISU AP rounds to penetrate OKW P4 with its 234 armor. That's all.
85% of the time at far range.
98% when it gets at med range.
On the same way you don't get a KT to kill tanks, primarily, you don't get an ISU to deal with vehicles. Both of this tanks fulfill the same role, slowly bleed out your opponent by dragging the fights into a slow grind since you are forcing your opponent to make some move. One can act as a spearhead/damage sponge, the other one pokes at AT gun lines from out or range.
ISU-152 has the G-530 Concrete Piercing Round that has plenty of penetration. I would start by lowering the MU cost of it and fixing it the ground collision a bit.
+1