It would surprise me a lot of Lillard got significantly better as an NBA player than he is right now.
I agree, I think he's mostly a finished product, though there's always room for mental growth, which matters for score-first guys (see: Pierce).
Lillard is a one-way star, like Carmelo, but he's still a really nice piece to have. I've said this sort of thing before, but I'd trade the whole Celts roster of role players, and all the picks coming down the pike, for just one young star like Lillard, Cousins, Jabari, Wiggins, Okafor, etc. It's always, always, always better to have that type of piece as your starting point.