Pontio,
Heres some info for you
1.8TQs have rear calipers and associated caliper carriers with a 41mm piston diameter acting on a 239mm x 9mm thick solid disc
S3s have 38mm piston diameter rear caliper (with i presume) a different caliper carrier acting on 256 x 22mm ventilated discs
If you want to upgrade to S3 rear setup then you can just source the 38mm calipers and carriers and its a bolt on solution. However, part of me wonders whether an S3 rear caliper CARRIER would work in conjunction with the existing 41mm diameter caliper of a 1.8TQ, while using the associated 256mm ventilated discs. My rudimentary knowledge of brakes suggests that even though its only 3mm difference and the rears do 20% of the braking at best...surely its better(and cheaper) to use the larger diameter piston caliper.
Anyoneelse care to comment.