Correct me if I'm wrong but I'm pretty sure SU-85 has differing TTK even vs panther, you yourself point out Jackson and FF have varying performance vs different tanks due to RoF and damage differences. Yet a repeated argument by some on this forum is all three units are apparently OP for the same reasons and lumped together in their calls for nerfs.
The SU-85 does have slightly better DPM (TTK vs Panther is ~28s from the first shot, disregarding accuracy and penetrations) but it has the obvious weakness of not having a turret against a diving Panther or other vehicles.
For what it's worth I think the Firefly is balanced (hits hard but slow reload and mediocre mobility), the SU-85 is mostly balanced (although I think the selfspotting is bad because it goes against the combined arms principle of the game, even if it has a significant drawback) except for its very high penetration, and the Jackson is slightly overperforming on purpose because it's the only real reliable late game AT for USF.
Obviously the SU-85's and Jackson's situation isn't ideal but tinkering with them will be very complex and have significant ripple effects, and that's not really something we want to touch at this point.
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T.R. TheSeekingGuy 
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