Athleticism is so overrated for power forwards. The position had the word "power" in it for a reason. The league is littered with PFs who are ath-a-letes who stink.
Sullinger has an incredible feel for the game of basketball. For PFs, that is rare. He has already shown that he's an all around player (score inside and out, rebound at both ends, outlet pass, pass in the half court, play team defense).
Once Sully is allowed more than a token touch or two in the low post, he will be a beast of a low post scorer. Then he will develop his perimeter game all the way out to the three point line.
I see Sully's upside as more than "solid starter" because of his feel for the game, competitiveness, smarts, and work ethic. It's a lot to put on a rookie, but the only way this roster has a chance against Miami in the playoffs is if Sullinger can make that small team pay by killing them in the paint.