he actually moves pretty decently, considering he's 6'6.
I think he'll be the most successful, because he has the most weapons. Maybe when Young goes through his major growth spurt he will develop some weapons, but when's the last time a guy won a Slam with no weapons either?
There are 2 issues that I always feel effect such tall players; movement and injuries. The tallest player to win a grand slam in recent times is Richard Krajicek 6'5, but his career was continually troubled by injury. It is the major question that I have against the exceptional Juan Martin Del Potro 6'5 although based on a video link on another thread he seems a very good mover, but can he avoid injuries.
Tennis players may be taller than average, but wait to see if the giants take over.