upgrade from 288 to 312mm is easy enough, all you need is new 312mm discs and a set of calliper carriers from something with the 312mm setup - otavia vrs, 180bhp bora, anniversary golf, s3, tt.
calipers themselves are the same.
cant comment on the extra stopping power, as ive the 312mm as standard, but its a common mod to do on those with the 288mm discs