500+ is a lot in a front wheel drive car.
At 400bhp I think the mk5 handles it very easy on a good set up, even without a diff. You've just got to know how to drive, it's not all about stamping your foot on the throttle like a heavy handed caveman.
Like has been said, sell your GTI, buy an Edition 30, a few mods will see 330bhp very easy.