I think it's fine except for it's pen. It has a very real chance of beating medium tanks 1v1 if it stays at range, which isn't hard.
I don't understand why a 50mm cannon has superior pen to the t34 / sherman (25% more, 120 pen vs 160)
First you speak of a scenario where it stays at range, then you take the penetration value from close range though. In long range it has a penetration value of 80 only, which is already lower than any other tank. In close range it will probably only fight if you a) outnumber the enemy or b) fight slow heavy tanks like ISU-152 and circle them.
If it is employed properly, why should it not be able to fight a medium tank? It is cheaper than one (of course not so much if you take into account OKW fuel handicap) and it has some strengths that keep the unit currently very powerful in the right hands (speed, range, smoke), but it also has several drawbacks in comparison to a medium tank (armor, health, poor accuracy and penetration on range, no anti-infantry capabilities, requires a lot more attention lacking elsewhere on battleground). I mean, the Puma is solely there to fight tanks and light vehicles, and for just light vehicles it would be too expensive (even though that is currently the only reasonable counter for American AA halftrack...), so it has to be able to fight medium tanks as well. And currently generalist medium tanks are able to beat more expensive dedicated anti-tank tanks like the Panther if flanked, so...
Anyway, nerfing the strengths of the units too much might turn the unit quickly pointless, because it wouldn't be able to perform well as a hit-and-run vehicle anymore. On the other hand it may be a bit too powerful for cost right now in capable hands (not so much in less capable ones...). Maybe a slight cost increase and a change to the veterancy ability could be reasonable, or a slight reducment in firing rate or even more reduced penetration so it takes longer to take down a medium tank unless you flank it. Or reduce the price significantly and make it only a counter to light vehicles unable to fight tanks on its own, but then OKW would have only very expensive anti-tank tanks left, which would cause other problems... especially since OKW doesn't even have a proper Anti-tank gun.