Are they a direct bolt-on? What's involved? Carriers / calipers / discs? I'm assuming the stub axels will have to be replaced with the S3 being AWD?

Why would you just upgrade the rears? Brakes are as much about equipment as they are about balance, if you get monster rear brakes (which in and of themselves may do nothing for stopping distances) and leave the fronts alone your stopping distance may increase.

Are you fading the current setup or are you just looking for a visual upgrade?

Depending on the setup it might take some plumbing changes and wheel revisions.

And want to balance out the rears. Wheel change is probably a given but I'm unsure if the swap is more involved than just carriers, and calipers.