Unfortunately for the Giants and the player, they are "guessing" at how he will do when he starts to experience "the competition."
To illustrate, I have somewhat followed a similar player who signed last year for well over $500,000. Lot's of accolades/can't miss/great power/etc. In 2 seasons in Rookie ball, the player is yet to hit over .175.
One common theme I have seen in some posts over the past several days is, my impresion at least, that high achievement in high school is meaningful in the world of baseball. Unfortunately, I just don't think it is. College and professional baseball has a way of exposing the weak spot of nearly every player, no matter how good they were in high school/college/elsewhere. When those weaknesses get exposed, some players never recover while others adjust and become better players.
Before you get to college and professional baseball, you really don't have a clue how you will compete against "the competition."