In my experience, moving the comb or the rib has minimal effect on point of impact for many shooters. Unfortunately, shotguns are not like rifles, where moving the rear sight or scope reticle has a profound effect on where the rifle bullet impacts the target. Yes, in theory, raising the shotgun's comb raises the point of impact, just like elevating the rear sight on a rifle raises the bullet's point of impact. However, in practice, this is not often true with the shotgun.
There are many reasons for this. One of the main reasons is when shotgun shooters shoot a moving target, they are focused on the target, not on aligning a front and mid bead (if there is a mid bead on the shotgun in question).
True, the beads may be aligned almost like rifle sights (in a figure eight, or similar), when the shotgun shooter calls for his clay, but at that point everything you know about rifle shooting (and thought you knew about shotgun shooting) goes out the window. Hence, the mystique of learning to shoot a shotgun well.
bluedsteel