And let say that ISU-152 does get such an ability and can now manhandle to Elefants and JTs from the front what would be the options for Ostheer/Okw to deal with ISU-152 then?
Elefant and JT can beat the ISU-152 for good reason.
Guaranteeing penning on shots doesnt mean the ISU will win, it'll still lose, but there wont be hilarious bounces from a massive shell landing on a Panther and then "dink".
That being said, I think I agree with you, broken clock being right twice and all that. if you got munitions to burn and add vehicle mark on top of it, you can just end the career of any tank of your choice. I would argue its better to keep the ISU's stats constant since lets be honest, its already an absolute monster when its well supported.
I'd rather the ISU get an ability that gives it actually useful utility and not necessarily just a straight stats boost.
But then again, when compared to the other heavy TD abilities like the Elefant's cursed stun shot, or the JT's busted HE barrage on top of their strong base stats, guaranteed piercing rounds wouldn't be completely unreasonable.