Dont forget the losses of a 4x4 transmission over 2wd. My mate had a 93 WRX Impreza with a front mount intercooler, exhaust with no cats, chip etc running 16 psi and he couldn't shake me when I was runningmy S2 at 18psi. Since then I've gained a bit more boost and the cars been on a diet.

I now have ample Scooby kills under my belt