I highly recommend Milltek exhausts, an Audi technician told me that it's the nicest sounding exhaust for these cars.
While the cat-back system was flexible enough to fit the 2WD and quattro models, it was only able to fit vehicles with manual (stick-shift) transmission due to the Tiptronic having a larger gearbox casing.
However, Milltek has now developed a downpipe specifically for the Tiptronic models (again, 2WD and quattro) and fits all models built from 2008 onwards. It features larger-bore pipework (at 70mm), T304 stainless steel construction, a choice of either dual-outlet 100mm tailpipes or quad 80mm tailpipes. As with all Milltek exhausts it's hand-build in the United Kingdom and is designed as a genuine performance-modification.
The complete cat-back system is available right now from your nearest Milltek stockist.