Mangum isn't going to be great... HE IS GREAT NOW. His numbers and RAT, playing against better opponents, is the same as C-Stews last year, and Stewart could have played professional ball in Canada. He's better than John Beck as a Junior and Max Hall as a Sophomore.
Mangum is great now. For heaven's sake stop living in what-if land and enjoy BYU having a great aerial-attack QB again!