Catalytic Converter, legalities Options

[Red_06_EvoRS]

I've been hearing ugly rattle/vibrations since the summer on my 93 GSX. I got out and listened and it sounded like it was coming from the catalytic converter area. It only does it when idling slow or chugging from a dead stop. I finally took it to a local shop and they said that I need a new catalytic converter (a couple of heat shields could also use some work).

I've always had in the back of my mind possibly putting on a larger exhaust, etc. etc. I know replacing the catalytic converter for no good reason is supposed to be illegal. So my question is, if I'm going to have to replace it anyway, is it legal to put in a larger/higher flowing one? Or does the stock one do the job well enough not to worry about it? I could see where the feds might say that if it's not replaced with an exact replica that it's not guaranteed to handle the smog duties that it's meant for. Anyone know?