Its also got 50 range? And costs 30+ more fuel than all the allied TDs?
50% durability advantage (theoretically higher with armor) and actual AI capability versus 17% range disadvantage and a 27ish% price increase doesn't sound bad at all.
This is added on the fact that the panther is about as hard a counter to tanks as you can get outside of doctrine in any faction, with no medium tank on the allied side having a snowball's chance in hell of fighting it. Casemates are all easily flanked and even a jackson can be caught out of position and destroyed by Panzer IVs. A panther, on the other hand, can literally outfight any medium tank in the game with its ass facing it and win through HP and DPS alone, let alone using the front armor, and has actual AI power to boot when it isnt zoning out enemy tanks.
The fact literally every single balance discussion regarding the Panther is dedicated solely to buffing its ability to counter its only realistic nondoc counters on the allied side is really telling of just how much this unit does not need a buff.
You can argue its not good enough versus heavies and maybe that'd hold true, specifically versus the IS2, but nobody's been asking for penetration increases so big doubt there.
End point: if you want this unit to be a better Jackson it needs to lose either AI or durability or some of both.