I just put some Magnaflow high flow cats on my car in conjunction with the Pro Chamber, and if you ask me the car sounds even better now than it did before. A lot of that may have to do with the fact that Im running 2 chamber flows, which supposedly sound better with cats installed. The car is quieter on the inside, but still sounds hella loud on the outside when you get on it (got to watch the mechannic get on it from the outside) With the cats the car sounds even deeper, and a lot more refined (sounds alot more like a 5.0 now) Also, havent really noticed any real power drop off. It seems a little more hesitant to rev to 6000 rpm (should be shifting at 5200 to 5500 anyways) but the car has picked some low end back up. Its alot easier to chirp (even breakloose) on the 1-2 shift. I didnt think I would be happy with this mod (adding emmisions devices to the car;they are really cracking down on cars out here that dont have cats. The cops will actually look under the car with a mirrior if you are pulled over, and if they dont find cats they impound the car. I didnt want to have to worry about that)but the results are a deeper, better sounding exhaust and not too much of a performance drop, if any.