There is more than one Karmann Ghia in my past Not quite the poor man's Porsche in terms of speed, but a very nice alternative at the time.

And last year as I was cursing the A class diesel with its poor power on the last hill of the A8 before the border, what passed me but an old timer Käfer! A little to my embarassment, but proof that my choice to be loyal to VW has a reason.

OP just be aware that your desire to drive at high speed may not be entirely compatable with an economy rental, as most vehicles in that category are not that high performance. I did have a 3 series BMW from Hertz last year but that was the only upgrade I can ever recall from them at MUC and I have top tier status. I believe that Sixt may be a better option for somewhat decent performance.