Seems like typical problems with cats are that they clog up. The exhaust prob is much less restricted now, so it should sound different, and since the cat is physical different in size, perhaps there is some resonant frequency created that makes it raspier than it was before. If you don't like the sound, you could change your muffler to a quieter one. As for the seafoam, I don't quite understand how it would clean the cat. Usually that stuff is either poured into your carb or an intake vacuum hose or poured in the gas tank. I believe I have read that some people may have gotten a clogged cat AFTER using seafoam b/c of all the gunk that it dislodged, burned, then passed into the exhaust. I haven't yet seen where it actually cleaned a cat.