The calipers are wider to account for the fact that the GTI disc is solid and thinner at 12mm IIRC as opposed to the R32/S3 which is 22mm wide. On the Mark 4 the R32 Vented/solid calipers all have 38mm pistons.
On the S3/R32 the pistons are a 1/2 way house between the 38mm piston of the GTI and the 43mm pistons of the similar RS4 as they areat 41mm so a little more hydraulic action going on there at the expense of a small amount of pedal travel.
They're a well regarded mod in general terms increasing stability, decreasing dive. The only people who dont really like them are the die hard track slaggers who prefer a little more instability in the rear to promote oversteer and better laptimes....not for the faint hearted though. In general for those of us with average ability and a desire to be able to drive to work Monday morning - bigger rears (IMVHO) are the way to go.
There is no commonality between the calipers carriers you need to upgrade both, pads are the same though
Plus they look better
