Correction to a previous post:
Not only the USA have emission laws and restriction on FreonR12-
Several European countries have endorsed similar laws and regulation. North of the border (called Canada) also have very strick emission laws, very similar to the California laws.
I can't see why there are some many people bashing about cat. This is not where the restriction is in an exhaust system. The biggest restriction is generally at the exhaust manifold and muffler end, not at the cat.
You can open/remove/enlarge the cat as wide as you wish, there will not be any significant performance increase, except the one ''perceived''