I cannot say for certain if I "know" it, but I may or may not "believe" it, thus possibly being truthful in that regard. However,factually speaking, I can't comment at this time. But will reserve the right to comment later... maybe.
I used to track a mk2 gti with oem brakes, ss lines, super blue brake fluid and pagid race blue pads... so no direct comparison to mk6 gti but with that minimum of an upgrade it was not an issue bringing down the car from 130mph down to 30-50mph.
Heat is the killer to brake systems and once you boil your brake fluid the performance decrease will drop dramatically. I'd start with brake fluid change and slightly upgraded pad (perhaps new set of oem rotors if they've become warped). During your braking tests remember to bed your pads properly and give enough cooling time in between each test.
