Simon, there is actually one person with B7 RS4 rotors on a MkV chassis. They're on a UK MkV R32, and APS have fitted them, along with RS6 8 piston calipers.
You can read about it here (cut & paste):
http://www.tyresmoke.net/ubbthreads...25,26,27,8/Number/1012181/an/0/page/0#1012181
Here's what that looks like (calipers painted MkV R32 blue):
The RS4 rotor is quite nice, but it isn't a fully-floating setup. Meaning that they will distort if their upper temp limit is exceeded, and that heat will still ne transfered to the bearings...
The RS6 caliper is an 8 piston / 8 pad setup (!!!)
It seems that the RS6 calipers need a larger master cylinder, as the R32's brake pedal now requires more travel before braking begins. Regardless, this is a very cool "OEM+" upgrade, that should prove effective on the street and the track.
Personnally, I will be going a different route, working with Alcon (UK) calipers and rotors, and STaSIS (USA) designed and fabricated rotor hubs and carriers.
These are Alcon J-slotted 370mm (14.5") rotors, solid one-piece calipers with four pistons, and both weigh less than half the standard items. They will fit beneath 18x8" ET45 wheels (Neuspeed forged RS series), without having to resort to any spacers.
STaSIS sell these brakes along with other engine and suspension mods as a package on US A3s, and the they are warrantied by Audi as such.
One-piece construction affords lighteness and stiffness. Piston sizes are suited to the A3's MC:
J-sloted rotors make for good feel and good pad wear caracteristics. Fully floating makes for great heat resistance, as the rotor doeasn't warp against the hub (another side benefit is that the wheel bearings' grease shouldn't vaporize as easily on the track - it's happened to me, thrice!).
I'll be fitting these next spring, along with a matching rear setup.
Yeah, so they're a bit big... (they also have them in saner 14" and 13.5" sizes, with even greater weight savings). LOL: