I don't think much over 300hp is worth considering, 320 on a Revo 2 is as far as I would go. I tried a twintercooler and upped boost and timing, yes it was quicker in a straight line but it REALLY reminded me that with front wheel drive and no LSD it's a compromised set up
P Zero's, TT arms, carefully set up suspension PSS10, WALK, neuspeed roll bars, bottom engine mount REVO 2 and the compromise is tolerable. I really didn't like the car with more power. Moving from an M5, then a well set up E92 M3 for me north of 320 HP is too much for the GTi, its time to look at other things after that, things with RWD or 4WD and LSD's IMO.