1) If you plan on keeping the car then do what you think suits you best. When I do my non-numbers matching Superbee I do plan on using something other than paint. My current DD (a new car) has a textured finish underneath and the stuff is really tough. I like it.
2) If you think you might sell the car in the future, document the snot out of it now. Take pictures of it before you paint the bottom to show what the metal looks like, then after the coating goes on. There shouldn't be any questions.
3) I would think the thicker coatings would also help with road noise.