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If you want a set-up that makes nice power consider a Dr. Gas x-pipe in place of the cats. Attatched to my TPIS headers this combo makes 343rwhp, 340rwtq on a motor that's stock except for ported throttlebody. Sounds great too.
Don't waste your money on high flow cats. Just do an x-pipe and some o2 sims and be done with it. This will sound much better and make better power. Only drawback is that you have to be selective where you go to get your inspections done.
I was skeptical too about catless gains until I did it. How it's done is important. Danny Popp, a national autocross champion has tested numerous header/exhaust setups on his Z and had excellent results by installing a Dr Gas x-pipe directly behind the TPIS headers. On his car he's making 402 rwhp with this exhaust, Vararam, tuning and a ported throttlebody. I decided to try it as I already had the headers and drive my car primarily on the track. Other than the exhaust my car is stock except for a hole in my airbox and a ported throttle body. With the new exhaust my car makes 343rwhp and 340rwtq. Though I didn't dyno it before the x-pipe it was probably making about 325-330 rwhp. I figure the x-pipe in place of the cats is worth 15-18 rwhp/tq and at w.o.t. the exhaust has a higher-pitched more exotic sound that sounds great on the track.