I think reducing the reversing speed of the Panther would make it really difficult to keep Axis armor in the field. The Jackson spam is already difficult to handle with Panthers, imagine if I cant reverse fast enough to escape.
don't put it in a situation where it can't; cover it with other units and use shot blockers. decreasing the reverse speed wouldn't suddenly make the tank over-extendable, it would just make it easier to do so. and really, it should apply to all tanks. although until i hear otherwise i'm going to assume it's not possible.