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).