Yeah Sully can play competent D. He's not a world beater but he's not a sieve either.
athleticism isn't everything. He can still find ways to compensate for his lack of athletic ability.
He can and from what I can see has. With the exception of Draymond Green (a complete and total revelation) he's the best PF from his draft class. You could make arguments for Henson or Jones but he was not only picked after those two guys, he's also got a strong statistical case as a better player than either.
If you're interested
here is the breakdown for that draft class of power forwards. I left off Davis off because he's not a PF, he's a freak of nature.
As for selections made after Sully, the only guy I might rather have is Draymond Green. But honestly if Sully improves his 3PT% (and he has steadily every year) I'm not even sure I would take Green over him. Seriously though, look through the bottom half of that draft. It was plain garbage. Ainge grabbing Sully was great and totally makes up for the whiff on Melo. Also I'm all for whiffing on 7ft guys. You seriously never know when Deandre Jordan or Rudy Gobert might just end up on your team.
Love Sully, hope he keeps getting better, hope we keep him for a long time.