I’m not aware of anyone who manufactured any performance-related turbo-back for these cars on a mass scale. Most are custom jobs. Diesels don’t usually have particularly complex turbo-back exhaust systems. The most performance to be gained is to create a simple catless downpipe and straight pipe setup, maybe with a resonator to address any drone that results.
There are aftermarket downpipes available from IDParts for the A3, not sure if there is a major difference in geometry between the B4 and A3 models but I reckon it would fit. I got the stainless downpipe from them and it is a nice piece, but I have a stock exhaust behind it.
If you can get away with it emissions-wise, get a muffler shop to build you a full straight piped system.