Nothing but respect and high hopes for Stras, but you can't rate him over a Gold Glove, Silver Slugger, All-Star Zimmerman when he has yet to pitch a single game in the majors.
It's a prospect rating, so if Strasburg turns out to be a better talent, he's right, if not, he isn't.
Of course another possible outcome would be that Strasburg has his career derailed by injury and we never can really make the comparison. (knock on wood)