I'll try to clarify some things first.

California has stricter emissions standards than the other 49 states, thus cats are separated this way and Cali cats are approved everywhere.

Permanently removing a working catalytic converter on a street driven vehicle is illegal. Replacing a damaged catalytic converter is perfectly legal.

There are many "high flow cats" out there. Some are advertised that way, but they are actually just resonated test pipes. Some high quality aftermarket high flow cats will pass emissions and will not throw a CEL (check engine light). Others won't pass inspection and will throw a CEL. Kinetix Racing makes a great high flow cat that has passed emissions, is guaranteed not to throw a CEL, and comes with a lifetime warranty. Berk also makes an excellent product, but I don't have as extensive knowledge on their product line just yet.