Are you constantly dying in the Panther when you kill those 20+ enemies?
If that's the case, then you might have a problem.
Dynamics of the game make it so if you stay out, you
will die. I don't retreat untill the fight is done, or I'm out of ammo/damaged with no resupply.
That being said, if you feel you have to run away in a Panther, so you can drive it again later, you're doing it wrong.
The goal of such a system would be to make the use of a perk tank an occasional special, not something that you're supposed to be able to sustain as your main ride. Trying to balance it so that the average sortie earns as many perks as it loses would break the idea of controlling the populations of perk tanks.
And I'm fine with that. But the Panther can't earn even 5 perks in 20 kills under typical conditions, and the people who get even 40+ kills are rarities. If the Panther is perked at 12-15, that means they won't earn perks unless they make something over 50 kills or so.
Now think about how many kills the Tiger II would have to make to break even. We're talking over 400, depending on price.
No harm in reducing the perk prices a bit.