i know this is kind of anecdotal math and with drag and stuff like that speed doesn't stay the same down range, but:

if you took 2 identical arrows shot out of identically tuned bows, but one with an initial 'horizontal' velocity of 270 and 1 of 263, all things being basically equal. at 90 yards the one going 270 would be 7 feet ahead of the one going 263. that's why i think sacrificing accuracy for a few fps just doesn't add up.