Depending on how you set it up, you could gain power or lose power. I'm not quite sure how much of an effect you'll see either way. The engines are designed to run at a set back pressure because the engineers design them with cats and mufflers in mind. "Mufflers" like glass packs (
http://auto.howstuffworks.com/muffler5.htm) reduce this back pressure and supposedly improve performance. However, if you install "dumps" before your cats, you can actually reduce your performance because the engine and headers aren't designed to run at such a low back pressure. It needs some sort of resistance to operate optimally. I know, it's counter intuitive.
Now, that being said, if you replace everything from your headers back with high flow components, you'll aleviate the differences in the system and get much better performance.
I know I gave you a longer answer than you were looking for but let me summarize with this: If you remove your muffler and keep your cats, you should not have any performance issues (other than your ears!) and maybe even notice an increase in performance. If you want to get rid of your cats and back, I would suggest looking into high flow components for your entire exhaust or at least your headers.