The main issue with the SU-85 was that it could spot for itself. The Panther can´t use its range to that extend. It should be this hard to penetrate frontally or it would remain a glorified Panzer IV. The same way you don´t attack a SU-85 with a Panzer IV frontally, you shouldn´t attack a Panther frontally and expect to win.
That was one issue, the other one was armor. Before the last nerf (Apr 24) the SU85 had a front armor value of 180 (p4 had very hard time penetrating). Since then it was reduced to the current 140.
High armor values should not be so prevalent in the game. They make game play stale when your AT guns' and tank destroyers' shots constantly bounce. And in the Panther's case it's even worse because the tank has good speed and high armor. That tank needs its cost/teching reduced not its armor increased.