I dont agree that the panther is a mix of mediocre characteristics.
Its very mobile, the heavy crush allows it to move around unhindered while being about as fast as any medium. Its armour is only reliably penned by dedicated td's. It has more range then any medium, it has more pen and more acc then any medium.
That is because you choose to compare it with medium tanks while it is a TD. I has less range then most allied TD, less penetration and less accuracy. On top of that it cost more and has higher pop.
Just by glancing at its cost and stats shows the panther is not meant to be the answer to every tank. Its meant to be a heavy counter. The stug and jp4 do a more then adaquate job vs meds. Even both p4 do a good job vs mediums then allied meds do vs p4,'s.
PzIV is good vs mediums but it is not got vs premium mediums and get rolf stop by heavy tanks(not super heavy).
The acc buff also negates the target size reduction only on the allied side.
Allied TDs get 3 times the bonus Panther gets while more accurate to begin with and while vetting faster.