Given what I saw out of Sully at Ohio State, I think its near miraculous that he was playing defense as well as he was this year. He will be a okay to good defender in the future but really would never be a defensive anchor type.
I also agree about needing size to be a defensive anchor. Its easier to do it with a true big man(KG, Duncan, Chandler, Bynum, Howard, etc.) but its possible, as Lebron/MJ has shown, to do it with a hugely talented wing that can also play unstoppable offense.