Looking at that pic on RSTT's, I'd say OEM Avus 17's would clear no problem.
Avus have MORE brake clearence horiztonally than RSTT's, and the difference radially between a 17 and an 18'' rim is 12.7mm, call it it.
There's definitely more than 13mm of radial clearance in the above pic. More like 20+.
The 996TT/R32 setup is pretty expensive though compared to the alternatives, so be sure you require that level of braking before commiting to the long term extra costs (initial outlay, plus future discs and pads)