I think his ceiling is around 19 pts, 48% shooting, 40% 3pts, 87% FT, 6.5 boards. To say he is obviously going to improve (on averaging double digit points none the less) because he was horrible is just a cop out. Many players (even #1 draft picks) have bad years and do not get better (Kwame Brown), in fact some get worse or just stay the same. To show improvement (especially when you already average double digits) is a good sign.
I believe that (by doing some research) the mean scoring average in the NBA was 8.3 points per game. Even though Bargnani was above that, that number is really statistically irrelevant because the vast majority of NBA players score under that. So a more relevant and telling number is the median scoring average in the NBA. This came out to 6.6 points a game. This means that exactly half of the players in the NBA scored less and half scored more. Bargnani was way ahead of the curve and did not even average 25 minutes a game. So at least statically he is ahead of most of the NBA in scoring, in only his second year (which was a poor one...)