It comes from the fact that the puma comes in so early that by the time it faces medium tanks it has multiple stripes of vets and puma vet is batshit retarded.Well, it can't come to the field any later due to how OKW teching works (unless it was moved to the Flak truck for some reason) so either slow down the rate it gets vet at (though I was under the impression this is tied to the unit costs and can't be tweaked easily ) OR reduce vet bonuses OR reduce base stats somehow to make it more difficult to stay in the fight against mediums while vetted.
Also with smoke, incredible acceleration and great speed, plus not so bad health there is quite a bit of room for error, not to mention a vet 1 puma can 1v1 a medium tank and win if it uses the ability, or cripple an heavy tank for the duration of the fight.
Smoke at vet1, disabling shot at vet2, actual stat boosts begin at vet3. Would that mitigate the problem? Of course tweak acceleration and speed on top of this if needed.