Zelazny,
My experience is that they can impact differently AND fly perfectly. Right now I' m shooting Muzzy 100 4-blades and they are impacting within an inch of my FP. I can throw on a Magnus Nugent blade and for whatever reason they will impact 6" away from that group....BUT they fly perfectly....Group tuned, paper tuned, done it all and the BH flys just as well as the FP.

In your first paragraph, you state that if both are flying perfectly and they are on the same arrow and the same weight they should impact the same. My FP are about a half inch long well my BH range anywhere from 1" to 2" long depending on which one I' m trying at the time. I think that this change in FOC can change POI but has little affect on the actual flight of the arrow. Can' t really explain but it in my experience, that' s what happens.