I couldn't find any other threads, for the MK3 at least, with any reliable information about bolt-on larger upgrades for the brakes. So I set about finding some information.
It seems that there is still a 288mm option officially listed for whatever reason (presumably just carried over from the MK2 VRS), but is not used in practice.
Though, 288mm isn't a great deal bigger than 256mm that come as standard, I also found that the Polo GTI comes with 312mm brakes on the front.
B