The heavy tanks destroyer will destroy mediums they simply will make them obsolete since they UP for Cost.
If the enemy call-in a Super heavy you can counter with the heavy tanks destroyer, if not medium tds is a more cost efficient solution.
Once more simply test the MOD.
If medium-armor level Tank Destroyers (StuG/JP4/SU-85/Firefly/Jackson) are meant to beat medium spam, then no change is needed. They already do.
If medium-armor level Tank Destroyers aren't meant to beat medium spam, then what is?