I highly recommend Milltek exhausts, an Audi technician told me that it's the nicest sounding exhaust for these cars.
The RS3 features the same 340PS 2.5L 5-cylinder turbocharged engine that was first seen in the Audi TT RS in 2009. Milltek purchased their own TT RS as a long term development vehicle and the results were spectacular - with a headline figure of around 450bhp with an ECU remap and a full Milltek turbo-back exhaust system.
The first prototype exhaust system left the Milltek development centre and was successfully tested with MTM UK (QSTuning). You can see the development system in action below. The car features a prototype full turbo-back exhaust system along with a prototype remap.