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I have it on good authority (AP Racing) that the Golf GTI does about 85% of its braking on the front wheels and consequently most big brake upgrades are front kits.
Oh, boy, you really do keep going on about that utter BS, from AP. As I have repeatly stated, the GTI, along with most road cars do NOT have an extreme bias like that. The Golf Mk5 platform cars are running around a 60:40 brake bias - and this is very EASY to prove!
For normal road use it's considered fine to leave the rear brakes as oem, but for trackday use it's advisable to at least upgrade your rear pads.
Nope, if you 'upgrade' the fronts, then the rears should also be upgraded, to maintain the balance. If you don't upgrade the rears, the rear end will become un-nervingly twitchy under heavy braking, in anything other than an arrow-straight direction, on billiard table smooth and level tarmac. And both you, and others have described this very situation.
If you don't, you may experience fade on them quite quickly and without fully realising.
That simply PROOVES just how hard the rear brakes are trying to work!!!!!! And it also proves that the rear bias is CONSIDERABLY more than the rubbish AP are spouting!
Unless you want it as eye-candy you don't really need to change the rear discs on the Mk5 GTI to something grooved and/or drilled - For the 15% work they do the plain discs are quite sufficient.
Nope, if you upgrade your fronts with 'bigger' discs, you really need enlarge the rear discs by the same relevent amount - to cope with the 40%, not 15% the rears have to deal with.
And you have completely failed to address the issue of the rear brakes working when the brake pedal isn't even pressed!