Switching to OBD1 is only really necessary if you have some serious engine mods. Plus it will most likely make you fail an emissions test because in the majority of states OBD2 vehicles are tested by scanning the ECU for fault codes.

On a relatively new car without a modified engine, a high flow cat might free up a few ponies but I wouldn't expect too much. Generally the more common things to do are an intake, header and exhaust. You can add a high flow cat to the mix and it might be a bit of an improvement, but mainly people run them if their stock cat is toast or they need to run much-larger-than-stock exhaust piping which would make the stock cat a big restriction.

As for a CEL, as long as the cat has bungs for both o2 sensors you should be fine.