Also why do we need the P4 to perform well vs Jackson? P4 can also bleed man power vs infantry as well. Multi role tank vs hard counter TD?
The SU-85 and JPIV are casemates. It's slow, has poor maneuverability and its turret is locked forward.
This forces to it to adopt a defensive playstyle and attack from range with support, as anything that gets behind it can kill it.
The Firefly follows a similar logic. It has a turret, but it turns so slowly it might as well be a case mate. Combine that with its slow rate of fire and it'll lose a close range fight with a faster, cheaper, more versatile medium tank.
So, once again, it adopts a defensive playstyle and attacks from range with support.
The Jackson has 640 HP, a turret with decent rotation, a good fire rate and good moving accuracy.
This means it can dive to secure kills
and take down medium tanks in a brawl.
This isn't something the other three 60 range tank destroyers can do. Combine that with it unaccountably having the best penetration (imo that should have gone to the slow firing Firefly) and it's no wonder the thing's an endless balance headache.
For the medium tank vs tank destroyer balance the other factions have to work, the Jackson needs some sort of disadvantage at close range. A 160 HP reduction and a corresponding drop in fuel cost is one way to do that.