If it is the cats I would go for something from Germany, like this, with metal element rather than ceramic. These are 400cell.
http://www.ebay.de/itm/Sportkat-400-...-/321751735732
You could go 200 cell if your car is pre-facelift since you dont have post-cat lambdas.
http://www.hjs-motorsport.de/produkt...talysator.html
I had two similar to these on my previous S8 always passed emissions, now I have two of these on my current S8
http://www.turbozentrum.de/0/a-31960/
There are these too also HJS a bit cheaper
https://www.turboperformance.shop/do...s-bis-2600-ccm