Dam I hate exhaust tips with a passion. If they did not have some type of product applied before any build up baked on its nearly impossible to remove. I'd try a stainless steel razor blade. Try to "shave" it off. IMO getting it to look metallic instead of black is all the customer is after in cases like these. If they were removable tips I'd suggest soaking them in something that would loosen carbon build up. You might be able to rig something up that would soak them, old plastic bottle cut up perhaps, and a little duct tape. Never know that's just what I'd try next.
