Balance might or might not be fine but having TDs that can hit and penetrate anything that is not a KT from range 60 with chances close to 100% is bad design resulting in reducing game diversity.
Reducing the effectiveness of long range TDs at max range will be an improvement to the game.
What diversity?
Allies have nothing else to contest panthers.
There is no diversity here, because there isn't meant to be any.
ATGs are unreliably to both damage and kill panthers and can't exactly chase even for a little, so something needs to pick up the slack.
So what diversity you're talking about?
Getting SU-76 or regular shermans against panthers instead?