Having uprated brakes will only invalidate your warranty on the bits youve changed. However, you will probably get a manfacuturers/fitters warranty for at least a year (see Nationwide). However, your not worried about your warranty when you get a remap?
Ive got 280 mm Black Diamond brake discs with Mintex pads - bloody powerful - cant fault them. Looking at around £120 (excl fitting) for the grooved and drilled ones (i have just grooved = £110), and then £70 for the pads (inc. fitting on the pads).
Go for the 312mm tt setup with cross drilled and grooved discs........ ive never had so much stopping power since I fitted mine.
Bought mine 2nd hand from a guy on the forum for £200 plus p&p 2nd hand. You can get the whole audi tt set up with standard discs for £250.