Don't think the data and numbers are going to be a good indicator of how good/bad someone is at heading the ball. Corners are very crowded, getting your head to the ball is not a good indicator, because it fails to consider how tightly the player is marked or the position of his body when he makes contact with the ball. Sometimes just getting your head to the ball is an achievement in itself, which then skews the data.
Just from the eye test, it does seem that Maguire misses chances that he should at least be hitting on target.