in the big picture, I agree, it probably isn't making a big difference. It does affect the shot cycle though as it changes the transfer of energy through the arrow as well as into the animal. also, the shock I'm referring to is to the arrow or shot cycle, not felt shock.
I agree, there's no guarantee an arrow will flex the same. my thought was that maybe that normal flex of the arrow also covers up some degree of imperfection, where a stiff arrow doesn't because like you say, we are controlling more of the arrows reaction. I don't really know.
I do know, like you, I also shoot stiff arrows and have accidently grabbed my sons arrows and had almost no difference in grouping. that's between a .300 and a .400 arrow and both arrows are close in weight, about 30gn lighter on the 400's. it was enough to make me think I should have stretched first lol. every arrow tunes different, so if you tune for a 250 then your groups should be the same as the 300 arrows that you initially tuned for.