well I dont think I'm even allowed to post information about this topic but...
The Audi A4 as GlastonbA4 said has been built such that of a poorly built exhaust system, the system originally in the car is Turbo -> Cat -> downpipe. The downpipe in the car is 1.78" for refernce purposes. VW 1.8T's are setup quite differently, their downpipe contains their catalytic converter elements, the poor part about the A4, is the turbo spools right up into an emissions process hindering performance greatly.
Most current exhaust systems are built cat back for the car. Clandestine hardware among others offer a different configuration, allowing you to remount an aftermarket catalytic converter element underneath the car and place what might be referred to as a "test pipe" in the space of the OEM catalytic converter elemenet.