I think, its going to take 50 grs, minimum, of head weight to see any difference. But you are only going to see the difference as far as trajectory out past 25 yards or more. If the arrows are spined correctly for the respective head weight, they will be just as accurate as far as groups go... if you could shoot them from a machine so you're not automatically compensating for arrow drop, the heavier arrow will have tight groups but they will be lower than the lighter arrow.
I noticed this back in the old days when I shot training wheel bows. 25grs would have no effect on sight pins, 50 or more grains, I had to move em because they hit low.