If you get a downpipe without the cat, you will have to get the rear O2 simulator, which is damn near as much as just getting the cat in the first place. When I got my RT downpipe with cat about 4 or 5 years ago, the O2 sensor was banging against the body of the car and was very annoying. I went ahead and got the O2 simulator to do away with it. I am guessing the ZZP pipe has the correct placement so the rear O2 will not do that. The RT downpipes are way to expensive for what you get now.