I think Sully can one day become the kind of shooter Boris Diaw is now.
Which is probably why your not a scout at any level. JK I am not optimistic about his shooting, it has been almost as bad as it was last year. Boris can drive Sully cannot, I do not ever think he will garner that versatility to be a shooting and jump shot threat.
Take a look at Diaw's shooting averages from outside early in his career. He's come a long way.
Still, Diaw is just a spot-up shooter from outside, not a high-leverage guy from that range. That's what I'm talking about. I think Sullinger can develop into a reliable threat spotting up outside, but as The Flex says, better for him to take those from above the break or the corner than straight away.
As for the back to basket stuff -- Sullinger has a couple of effective moves. But he's light years away from being the kind of threat, with the kinds of moves, that Zach Randolph is and has been. Sully is never going to score 20 a game getting fed in the post possession after possession. Z-Bo has been a go-to guy in the post for much of his career.
As a post threat, Sullinger is more similar to somebody like David Lee or Carl Landry. Which is to say, it's a weapon he has, but not a major one (and Sully doesn't have the athleticism of either Lee or Landry).
By the way, it's rather obvious that Sullinger's shooting percentages inside are better than those outside. That's true of nearly every player in the league. More interesting would be how many points per possession the Celtics get from Sullinger working in the post versus taking jumpshots from 15-20 feet or further.