As others have said, you probably won't like what they end up sounding like. A buddy of mine did the same on his sporty, he ended up trashing the stocks and getting slip-ons. There is a large variety available and prices vary as well.
I guess I'll be the first and maybe the only to disagree with all the other posters. I drilled a 1 1/4 hole (if my memory is right) threw the center plate in my stock mufflers and they sound great. Others have said my bike sounds good and I've done this mod for a couple of friends rides and they're real happy with the sound. They look the same as the inside of Reinhart mufflers now. The bike is louder now but not obnoxious. It will bark if I twist the throttle though. Anybody want a set of poor sounding Cobra's?