Jaren Hall has been fine so far. Not great. He has struggled with accuracy issues and he's holding the ball too long. His greatest asset so far has been his feet, but he's been reluctant to use that asset with regularity. It has been a huge strong point in key moments in BYU's games so far, but the trade off to date has been in a mediocre passing game.
If Hall weren't available, I'm not sure the drop off would be all that noticeable. The passing game would improve, the QB run game would be worse. But I think BYU has so many weapons in the passing game that it's arguably a net improvement to make that area stronger even if you lose QB mobility.
I really like Hall. I like his attitude, I like his mobility. And as the season progresses, his passing skills and decision making may improve leading to quicker throws and more accuracy. But at this point in the season, I think Romney would be equally adequate as a starter.