The stock cat doesn't pose much of a restriction. If your cat is old and has a lot of miles on it though, all the trapped smog particles do eventually slow things down. It's like a sponge, it can only suck up so much before it fills up.

As long as your car is over 5 years or 50,000 miles you can legally replace the cat with a "high flow" unit from companies like Carsound, Landspeed Racing or Random Technology.