I'd say 280's will be fine up to 300bhp easily, 20mm won't make much difference at all, just buy very good discs and pads and you'll be fine.
I ran 280's with S2 twin pots, EBC discs and green stuff pads, G60 with 220+bhp that got stick daily, brakes never once faded, glazed or got pedal travel. Mine would hit 140mph easy, a 1.8T will probably see you over 150 but I wouldn't want to be doing that very often at all in a mk2
