Everyone makes catted X pipes, Bassani does, Pypes does, I think BBK does, SLP does, I think Kooks does, plenty of them out there... Or for $150 you can buy a pair of pypes cats and make your X pipe catted.
I'm not sure how Californistan does its inspections but if all they do is the OBDII check without a sniffer you should be fine as long as the car isn't throwing codes and it shouldn't
I have a Bassani catted x on my 91' and it passes Cali smog with sniffer. They guy just peaks under and sees cats and runs the test... Im guessing it would pass easliy with a newer car.
obd2 includes code scan, visual and tailpipe. none of the high flow cats are legal in CA for 96+ mustangs. keep your factory pipe and swap it out when you get smogged.
Its not the smog im worried about ! They have this new City Cop team that is going around and if your car even looks modded the pull you over, check your engine, exhaust everything ! Its kinda FCKD UP, but its Cali so its not surprising. I just dont want to the hassle if I get pulled over !..
I wouldn't worry too much. I am also in Cali and I have aftermarket high flow cats. I could be wrong, but I believe most cops are more worried about the import crew. Just don't act like a jackass and you should be fine.