I really don't think it's his IQ that holds him back. It's not like the guy is Del Zotto. He just doesn't have an overall elite skill set. He has an elite skill in his speed that blows away most professional players, but that's where that elite skill ends.
He's not a particularly good stickhandler, he can't really shoot a backhand, his wrist and slap shot are both relatively weak, etc.
Thing is, he has taken that single elite piece of his game and formed the rest of his game around it. It's a very smart thing to do and one that helps him to be as effective as he can be.
He's a fantastic penalty killer, he's great at pushing the pace and helping other players open up space, he just can't finish to save his life. It's far more relative to his overall skill set though then his IQ. Frankly I think Hags is quite a smart player.