One factor about SL that no one has mentioned so far in this thread is that the offenses aren't usually run with the proper spacing like in the NBA.
When Sully gets touches in the NBA, it will be either out on the floor against bigger covers, or in the low post against smaller ones, both situations where he can get something done. When he drives or posts, there should be more room to operate, with the Cs usually having good shooters on the floor, well spaced for threes. Sully has shown a great feel for passing, and will learn to give the ball up for open jumpers when his defender gets help, either on the drive or in the low post (something Bass can not do).
If Sully can play real NBA minutes this season, it's going to make Boston so much better, since he's a bull in the paint on offense, and a defensive rebounder with a good body for boxing out and a good feel for where the ball is going to carom off the rim.
I think the Cs brass feel that Sullinger is a player with a great feel for the game. He's shown that feel for the game at every level, despite the same doubts that are outlined in this thread. If they're right and he can start alongside KG, that will help both of them tremendously, with Sully mucking up the paint, and KG providing both a potent jumper to dissuade his man from doubling on Sully, and defensive cover to help Sully play NBA defense.
Put me in the "Sully can start" camp, despite his low FG% in SL. In that "5th Beatle" spot there are a ton of easy touches. Perk could have averaged 16 PPG if he could have caught the ball and didn't have a pump fake fetish. Bass gets a ton of open looks. Sully will get a lot of layups and open 15 footers if he can earn the starters' spot.