Perhaps I have a maximum utility to happiness, which increasing happiness approaches asymptotically?

Yes, I think that's quite possible, but I don't know whether it's actually the case or not. A big question I have is whether any of our values scales up to the size of the universe, in other words, doesn't asymptotically approach an upper bound well before we used up the resources in the universe. See also my latest post http://lesswrong.com/lw/1oj/complexity_of_value_complexity_of_outcome/ where I talk about some related ideas.