Yes, they both absolutely can get better.
Of the two, I think Sully is the most likely to be a good to very good NBA player. I've liked what I've seen out of him of late. I was impressed with the way he battled through his shooting slump, and I thought his defensive effort and execution has shown a bit of an improvement of late. Additionally, he is developing a pretty reliable fadeaway jumper in the post, and if he can come up with a counterpunch to it he'll have two post moves he can go to against almost anyone down low. The biggest improvement he needs to make is with his body. He needs to lose 10 - 20 lbs. to improve his quickness and mobility. I feel like he should watch a lot of Paul Millsap highlights (young and old) as Paul is a very successful undersized PF that I think Sully could model a lot of his play after.
KO for me is all mental. If he can play like the aggressive player we've seen 5 - 10 times this year he can be a real impact player for the Celtics. He's more athletic than I think he gets credit for and he is very skilled, but he is tentative and passive too often. I feel like he's going through a bit of a slump like Sully did earlier in the year, so it will be interesting to see how well he battles through it and continues to improve.