The StuG and JPIV are casemates. Penetrating frontally is all they can do. When they can't, they become worthless.
If you see your enemy getting T70 and nothing else you know it´s a IS2/KV2 stall and you prepare by getting a Panther unless you can obliterate your enemy before 13 CP and it doesn't matter.
If you see your enemy getting literally any other tank and you feel the need for mobile AT you use Stugs or JPV and they do fine.
What is the problem here? IS2 is the only allied tank with really high armor, and it´s doctrinal. So just because you can´t use Stugs and JP4s to counter it it must be nerfed? How does this make any sense?
The way Panthers and Stugs/JP4s are balanced right now is as close to perfect as it gets IMO. The only thing I would change is give the JP4 a vet1 timed ability to trade ROF for penetration. That would be fair considering it's price tag. The Stug on the other hand is totally fine.