Could he become a star? Certainly. He could also develop into a fourth outfielder who has a long career due to his speed and defense but who never hits enough to justify full-time play.
This is part of my frustration with distilling a player down to a single letter grade. Some "Grade B+" guys are virtually sure things, with high floors. But some are guys who could be stars, or who could turn into nothing at all (Rymer Liriano of the Padres is another example).