If you are installing long headers I have read on here that it is a VERY fit in your bodystyle(but would be tight in any late model mustang). Cats don't really rob more than say 1 or 2 horsepower at higher rpms. I'm surprised you don't have to keep them on your car in Washington. I've never heard of cats not being enforced. I live in Jersey and not only is it a huge fine for getting caught not running them, but the cops could have your car towed if they want and you'd never pass inspection here either.