Why not consider something "made in Pfaffenhausen"? getting RUF 3400, 3600 or RK Spyder instead? The Spyder has 420 BHP and kicks serious butt.
Only problem is it's only built in 49 ex. and probably costs a bundle.

www.ruf-automobile.de

If you go for tuning, I wouldn't go for a supercharger, but conventional tuning via ECU, cooling, camshafts, air intake, exhaust, incresing stroke, fuel pump etc etc