If you want to push the gti engine as far as you can....
Port and polish cylinder head, and fit with high lift cams
Fit a good 4-2-1 manifold, decat and straight through system to a high flow back box
Fit lightened fly wheel and uprated clutch while your there (if your into that sort of thing, but it wont make enough power to need a clutch)
Chip the ECU if you can find a decent chip for it
Fit adjustable fuel pressure regulator and ramp it up a bit.
The engine will feel more responsive, much more revvy, but be warned with the high lift cams, the engine will run lumpy at idle, if you hold the valves open long enough it will want to stall at idle, thats fine but with a light flywheel, you might need to permanently increase idle.
Fit a good cold air induction kit.
Expect 120-130bhp depending on how well you did it and how far you pushed it.
Oh and with all this your MPG will probably be worse than a VR6
