Post is from quite a while ago but TTE 420 is a K04 hybrid. On a MK6 GTI that was otherwise stock you would upgrade theintake, go to a 3in turboback exhaust, clutch upgrade if you are manual, upgraded intercooler, upgraded tubing (better flowing) tubing between turbo and intercooler and between intercooler and throttle body. Probably would put in new sparkplugs that are colder and gapped a bit tighter. And of course you would need to have the car tuned, probably custom because I don't think there are any off the shelf tunes available for that limited production turbo and tuner will probably want a 3 bar MAP installed.
Probably 4-5K in parts/software plus the turbo plus labor. You could expect to make 400whp.