I've never used one, but it'll definately help if you have other exhaust mods, such as larger piping, hi-flow muffler, headers, air intake, anything to help the air flow easier and faster. If you just get a new cat, it won't help as much, but it should still help some. I have one being shipped to me, and other parts to my exhaust on the way. I'll let you know what it's like when I get it.
Edit: I didn't think to look and see what forum this question was in. The headers I'm getting are for my Corolla, not MR2.