Im not sure on how restrictive they are. But I can tell you that if you hollow it out it will make a completely different sound they with just a straight pipe. A friend with a new fo#d hollowed out his cat and took the muffler off and it will clack and crack, I dont like the sound of it all the much. If I where to hollow out the cat to make it "LOOK" legal I would run a pipe threw it so it won't have a place to echo off of like that fo#d, hope that helps a little.
Justin<><