From what I've seen he's very comfortable and good the ball, but what I haven't seen is if his size makes him vulnerable to small agile players on weaving runs, the way Mangala is.
I think Jonathan Tah is even more highly rated in Germany, and he's a year younger but already leapfrogged Süle into the German senior team. He's 6'4 and still a unit, but seems more graceful in the way he moves.
The thing with both of these two is we'd probably get both for the price of VVD, and assuming we are even looking for a new CB, it's going to be a choice between potential or immediate impact again.