If you remove the cats, keep them for re-install at a time when you go to sell the Mustang or trade it in. It's been discussed here on this site many times; federal law supersedes any local or state laws. Federal law prohibits the removal of them, so a police officer in a bad mood can pull you and fine you. If you go to seel the car, a dealer will have to install them in order to resell the car. If you sell it privately, you'll need to re-install them ti follow the law.
But many don't, and get away with it. Its your call.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.