That is a picture that I found Facebook and that downpipe is huge. I am going to guess somewhere around 4-5" OD. Probably a good guess that just gutting the stock cats would be more than enough. Still interested to see what the catback size is though. If this is anything like the B8/8.5 though then there probably is not much more power to be made from an aftermarket exhaust.
EDIT: Had to correct my autocorrect.