A 100 grain difference will not only yield a different vertical impact at further distances, it will likely also lead to a different horizontal impact as well. The shaft that is 100 grains heavier has the effect of reducing the dynamic spine, which would cause it to impact right of the lighter arrow. These differences are there and measurable, but probably not terribly large, particularly at closer ranges, i. e. less than20 yards. As Ragnarok Forge notes, most probably can't shoot the difference.