I have seen Sean up close since he was 11-12, he is a great kid, but not LDS.
Having seen him play a lot, I think his playing style is most similar to Jaquizz Rodgers and I don't expect him to get much bigger, his parents are pretty small, and he has always been about the smallest kid on his football teams, and the fastest, but he will be lucky to get to 5-9 as 5-7 is already a "in his tallest cleats" measurement IMHO. That didn't really hinder Rodgers and I expect it won't hinder McGrew, because he is a very similar runner, he squeezes through small holes and makes great cut backs in addition to having the elite top end speed, he is very difficult to stop when he is in open space.
All that being said, McGrew would be a good fit in a BYU type offense, and I think he would fit just fine in the BYU atmosphere, and he is a pretty good student, but knowing the McGrew's I have to think that the 3 Big California schools will be tough to beat out, as I expect Sark to pop up at some point, they would have a lot of personal connections.