To see which karts author should be contacted to improve their work and integrate it in STK, I tested various extensions karts.
Sadly most are really too far from STK standards, even these of 0.8.1.
So here are the few which are not too bad [they look better in game than on the screenshoots] :
Penny & Punny
Punny is pretty much a copy/paste from penny.
The minus with them is that their little auto doesn't go well with karts. However they are pretty well done.
Minix
Based on a F1 of the 60's, he's really good and has funny animations, but definitely need graphical improvement.
Alternative Tux
I don't like him so much, however the kart itself is well done, way better than pretty much all other.
Python
It isn't that much a character so I don't think it can any way make its way into release. However he's really well done, so I felt I had to post it.