Sullinger is at best a bench guy on a contending team, unless the four other guys are HOF or something. If he were in better shape it might change things, but I don't see him ever being athletic enough to switch to the 3 full time like Millsap and at the 4 he'll always be undersized and a defensive sieve. Not a bad player, just probably not worth overpaying for in 2016.
It may also be a case of having to be traded or suffer a bigger setback for him to get the kick in the butt needed to change his habits. Gerald Green didn't get it together until he was playing in Russia and realized he had to get his act together.
The problem is that necessary setback is not going to happen in Boston with nobody that good really competing with him for the starting job. He's like a smart but lazy kid just skating by at school and getting Bs. Why should he get in shape when he can probably already get a 10-13 million dollar (conservative estimate) offer in 2016 when the cap rises?