Correct me if I'm wrong, facepulling only remodels the maxilla to be more forward (and/or upward), not lengthen it in anyway. What would happen theoretically if you reach your genetic potential for facial development and continue facepulling? Would your maxilla remodel to be more forwards or would it actually lengthen? On the same note, although maybe not entirely right, do you think Weston Price had a point with diet/nutrition and facial development? In other words, do people with proper nutrition also have a longer maxilla/mandible?
Are you asking if facepulling only shifts the maxilla up and forward along suture lines or if it causes bone growth? To be honest, I can't say with any authority what is going on, but I think it is mostly repositioning in the same way that expanding the maxillary arch relies on separation of the midpalatal suture, but there might be some component of bone growth/remodeling stimulated by the traction. I suspect the closer you get to ideal form, the harder it becomes to induce change. I discussed Weston Price and other diets in the thread called "Nutrition."