I think Sullinger would be GREAT next to KG. And if he slides, I would try to trade up for him. But I don't think its in the cards.
As for what I actually think he will be, I think it will be somewhere in between Kenny Thomas and Elton Brand. I think he brings more intangibles and defense than Thomas, and has similar offense/rebounding to both of them, but he isn't as exposive as Brand was when he was younger (although, I wouldn't even rule that out, once he gets with an NBA trainer/HGH dealer).