May I suggest, if you are going to hollow out your cat, that you just go ahead and put a piece of straight pipe through the middle of it. If you are only leaving it there to pass the "hey look, the cat is there" test, then put a pipe through it. Personally, on my "other" car, I just put flanges on the exhaust and put an "off road" pipe on. When it goes for inspection, I switch the cat back in.
Either way, the reason for the pipe through it, is that, when you hollow out a cat, you are creating some very odd turbulence issues. This may actually hurt your performance.