Now while I agree that if he improves as much as you state, he would make a ton of money and it would look better.
However, if he stays the same, no chance he goes 15th in the draft. Forget about the rest of the draft board. A PG who can't hit a jumper and is 23 years old is not someone a GM takes in that range of the draft. A junior who hasn't developed a jumper will never develop a jumper. He falls to the late first round (which could be a good thing for him from a bball perspective, but not from a wallet perspective).
Now, lets say the unthinkable happens and he tears his ACL (people point to the Ware injury, but the Noel injury will be the biggest factor for anyone). Teams won't draft him in the 1st round.
If you can get a guaranteed contract in the NBA, you HAVE to jump, IMO. People talk about Donte Greene, but he still got paid.
If you fail in the NBA, you can always go to Europe and make money. It is always better to go to Europe after failing as a 1st round pick then failing as a non-guaranteed 2nd rounder.