Sylvan , you are correct sir .
It doesn't matter if you shoot a 330 grain arrow or a 530 grain arrow out of your bow - the initial kinetic energy (from 0-30 yards)that the arrow will have just won't be that much different (generally <4ft-lbs). If you don't believe me, get out the calculator and chrony.
True and the 4 lb gain comes from the heaver arrow , the original posters question . It is relevant if you want to hunt Africa , they have minimum ke requirements .