Svein,
is your GRP-A a Turbo back? Does you car show fault codes now you have no cat??
Alright 20VT, me personnaly, I like to be able to hear that I spent around £500 quid as well as see it hangin out the back! Think Milteks are too quite, also heard they sit squinty (rear box hangs down more on one side). Personnaly I'd go for the new APR 3" back, then remap the car after fitting as current remaps don't tend to like the extra flow offer by the new down pipe.
As Dan said, S3's don't tend to get anything out of just a CAT back replacement.
Hope this helps. Happy shopping!