Well I've pretty much became an expert here. I know you remember all the shit I went through. Oh yeah, I ended up having to pay the fine too. Regardless of my correction. Apparently a CARB violation is considered a NON-correctable violation. Anyways, last I heard from a muffler shop owner, december 04. He carried aftermarket OBD2 cats, but they were not CARB approved for use in CA. He stated that there has not been an OBD2 cat for use in CA that has been approved. This guy actually told me, look dude I ain't gonna take your money, it sounds like you have a problem that should be covered under a warranty and it was. An 80k emissions warranty. Now it all depends on who is looking at your car too. Most places just stick a mirror under there and see if it's there. They're not trying to make love to the damn thing. Even at the BAR Referee station, I watched every move the guy made, and even he just did the mirror thing.
My brother has a 96 RS. Is that OBD1 or 2? Cause he bought a factory re-manufactuered cat and that worked great for him and was 3 bills installed. So that it a possibility too. Also the heat shield on the stock cat comes off on both sides with a 10mm socket. It might be possible to install that around an aftermarket cat to give it more of a stock look if that is what your concerned with in addition to it being functional as well.
Hope all this helped. Good luck man!!!