Players absolutely develop in the NBA. Now, you are not going to find guys who go from nothing to elite in a particular skill. So, Anthony Davis is probably never going to have Tim Duncan's skill set. Those elite talents are either genetic or learned early. But, players can develop certain parts of their game. Bruce Bowen learned to hit 3s. Ewings entire offensive game was learned in the pros. Just playing against elite night in and not out improves your game.
I think Sullinger can fit into the Clarence Weatherspoon role of 10 and 10. He's clever around the rim and has a good nose for rebounds. But, his playing time will be dictated by his defense. If he can't defend opposing 4s, he won't play. How does he defend Amare, Durant, Nowitzki, etc... That is a major problem.