When you upgrade your brakes you should change your master cylinder to the larger mm item.I do believe it is a 22.6mm you want.I cant remember but if you look at that link to my build thread i gave you the other day it is in there.
A later spec gti one bolts straight on,but if you go for a 2nd hand one make sure it hasnt got the output for a hydraulic clutch slave cylinder or you will have to blank it off.