It may be powerful, but very, very often shells cannot reach infantry because they end up in the ground somewhere in the middle. Not saying it's a problem - just saying it's not alaways possible to use it efficiently on each map.
Countering it as Ostheer? Many ways to go...
Elephnat (2 great doctrines), Pak43 (at least 1 very useful), CAS, Lightning - all work quite good agasint it.
You can even try Panther/Stugs with sandbags since they increase range.
The ISU not being usefull on certain maps strikes me as fine, seeing as most units aren't usefull on every map.
Pak43 gets hardcountered by both ISU's commanders with 1 simple click (yee for great commander design) CAS (as in, the commander) gets hardcountered by ISU with micro, whislt panther and stugs with sandbags.... well at guns exist for a reason.
Only elephant/JT can fight ISU on a semi-even terms and even then it takes a wopping 27 seconds of continuous fire from the ele/JT to kill the ISU.
Plenty of time to gtfo.
Without those units, ISU still wins games with a little at support.
Don't buff it, it's fine.