That's the usual wisdom of X-pipe vs H-pipe (that X-pipe surrenders some low end torque).
However the one I have been looking at that was developed for Porsche 928 seems to increase torque across the whole range.
http://www.performance928.com/cgi-bi...ss_parent=1128
But on the other hand will it work well with Audi's 4.2 V8? How different is the 928's V8 from the Audi V8? who knows.
I would think any X-pipe works better before the cats, not sure that the cross-over can pull the gasses from opposite bank as well through the mesh of the cat. I'm not sure if there is enough room in the D2 to put it before the cats.
I'd have to disagree about the sound Mikki, I had a x-pipe on my previous S8 it sounded gorgeous. Also just check out some of them on youtube, they sound very good.
https://www.youtube.com/watch?v=QbaSAxgxtPc
https://www.youtube.com/watch?v=nqrufMqTQpY