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From what I've read here, on a mostly stock (bolt ons) engine, the gains are minimal (~5hp) if any. Some people see gains, some see losses.
It will make the car considerably louder, and many people say the exhaust smells a lot w/o the cats.
I don't think it's worth the effort, but then again, I already have 02 sims, so I may give the ORP's a try next time it's on a lift just to see.