I've cored a set of Monster exhausts. Unless you really, really want to do it yourself, Strati offers a real bargain for doing your pipes for you and he's very experienced at it.
Personally, I don't like the sound of the ones with most of the insides removed. I cut about 3 inches off of mine and rearranged some of the internal paths to get better flow, but kept the small outlet on the tip. I'm happy with both the sound and performance of mine. The fully cored and shortened pipes are loud, but don't sound as nice as the carbon fiber slipons that I used to have. My current pipes are about half way between the stock sound and the slipons, which was what I was aiming for.
If you just core your pipes, or even get slipons, without modifying the airbox, it will be hard to tell any difference in power. Generally, you can get away with modified pipes without remapping or rejetting.
When I cored my cans, I spent several hours at it and used up about 10 hacksaw blades doing it. Obviously it's better if you have a bandsaw or some other good power tools available.