Legal: Above plus performance chip, supercharger, sho-shop Y (not the off-road), blah-blah. Basically anything that doesn't delete any of the three cat canisters. There may be a law about replacing the stock cat before 50k miles, but somebody else will have to chime in on that one.
Illegal, but you could probably get away with: Headers, gutting pre-cats (but not the main). Theory is, by the time you get to the check station, the main cat is warm enough to be working.
Right out: "Off-road" Y-pipes, gutting main cat, etc.
