Gutting your cat creats a fuckload of turbulence in your exhaust... You know what a cat looks like on the inside, right?
A metal screen... if you gut a cat, and don't get EVERYTHING out, you A) remove torque-adding backpressure at low RPMs, and B) will create turbulence that will slow down your exhaust at high RPMs.

While, y'know, that metal screen just restricts the airflow that much... it's pretty much pointless unless you've upgraded your entire exhaust system, and a high flow cat will always give you better results, with only a test pipe getting better numbers (~1-2 hp over a high-flow cat... and should only be used in track racing conditions).

Also gutting your cat will give you a horrible ricey sound and a horrible stench. I can smell a gutted cat about 100 feet down the road...