I think Sully has more upside than many people think. Regarding rebounding, you don't have to be taller than Sullinger to be a good rebounder. This year four of the top six rpg guys are Sully's height, Varegao, Zach Randolph, David Lee, and JJ Hickson.
If Sully can become a 11-12 rpg player, and develops his offensive game, he could be an all star level player. He has a good shooting tough that extends to the three point line, and has the knack of getting his shot off in the paint against bigger players. He also has a very good feel for the game, even as a rookie.