Personally, I think there may be many factors to be considered here.

1: What the skill rating for the defender in "heading" (not necessarily SA).
2: What the skill rating for the attacker in "heading"
3: What the skill rating for the corner taker in "crossing".

I have four players with corner SA, in my squad. But by far and away, the weakest of them (in terms of quality), has the most success in setting up goals, because his "crossing" points are higher than the others.
Likewise, my top scorer from corners, is not my top rated striker. When these two play together, I'm always pretty confident of getting at least one goal from the set piece. Any other combination, and its 50/50.