I have a WRX downpipe on my STi
It did not fit directly in because of the bracket on the drivers side of the DP, the STi tranny is larger and that bracket needed to go in order for it to fit. Other than that it bolted right on.
I wish I thought about comparing the exact size of the cats, but it was the last thing I was worried about at the time. We checked the diameter of the pipes and they looked identicle. In it went and its been on since.
Cheezy video here -
http://forums.nasioc.com/forums/show...hreadid=549344
fwiw the car is not FASTER, but it is quicker. I think it is due to my very restrictive muffler. Either way it sounds mean as hell
~v6