I've changed my mind. Rather than going for an obscenely weird car that's massive (Tank Car) or has outlandish flaps (HSR-II), I'm going to try and tune up a Spec Miata to more-or-less what I'd do to a real one when I get the chance.
Works out at about 263BHP/ton at the minute (I think - it weighs 1228 and has 324BHP, that sound about right?). And actually it's not half bad, surprisingly enough.
I was about fit to give up on the Spec Miata - I wanted a Barrybonnet on it and I wanted it supercharged, but I couldn't get the pace out of the two 1.6 Premium MX-5s - I could get a 1'15.4 at best. After about a billion hours of suspension fine-tuning, one click of rear brake bias gave me a 14.6 with the stage 3 turbo. Back to the supercharger and I've lost just enough power to lose almost exactly 0.4s on the straights - a massively overdriven lap gives me a 1'15.027 and that's my car