depending on how often you have to get your emissions checked...in az you have a choice to go for 1 year or 2 years....my friend just kept his stock muffler and piping...and just changes it when he has to go in...works for him...couple hours of work every two years can't hurt..
I'm no tree hugger or anything but why don't you just leave them in there and get some high flow cats. Taking those cats out just so you can gain maybe five horsepower is really bad for the enviroment. Spend the 200.00 dollars for the cats.
Hi flow cats may be considered emissions illegal as well...
You're really only allowed to replace cats with the exact same thing as what came stock on the car. That's at least for CA emissions. The strictest around!