You are not going to gain 10 HP from aliminating them. I doubt that you will gain more than a couple of horses which you will not be able to feel anyway. Plus, your car will not pass emissions.

But the best way to do that, is to cut down the cat canisters and weld straight pipes. You take out the header, cut the cats off and then take the header to someone to weld the pipes.

If you leave the canisters on and just brake off the material inside, you are not going to gain any horsepower because of the decreased exhaust gas flow velocity and turbulence that the empty canisters will provide. As a matter of fact, if you do that, you will probably loose a couple of horsepower.