I remove the cats, and put 2 2.5inch tubes all the way to the rear silencer.
I did it in diferent steps, first the cats, 1 was tottaly bloked, the car run better and fuel was half it use to eat.......so 2 big tubes all the way to the rear.
A lot of noise
, much more fuel economy, and run like it have 40hp more.
a8 97 4.2 abz