As someone who has been touting Jajuan Johnson the last month on these boards (maybe half of my posts are devoted to him) I must say I'm extremely excited. He's a great defender (he blocked more shots than he committed fouls!) a scorer, athletic, and a team player. He worked on his game a lot every year in school, be it at the free throw line, extending the range of his jumper, or his defense. There's no reason to believe he's not the type of kid to continue to work hard and get better at this level too.
That said, he's strong, but he's probably not going to put on all that much more weight. Maybe the Celtics will do him a favor and list him at 10 pounds more than he is, but some people are just stringier than others. Given all the reps he did at the combine, it's clear he's hit the weight room a lot, so he's probably done growing. I mean, given all his great attributes, if teams believed he could add another 15 pounds of muscle, he would have gone far earlier than #27. So let's let Jajuan be Jajuan.