A certain bow makes " X" amount of energy period. Going from a finely tuned arrow at 10 grains per # to a finely tuned arrow at 5 grains per pound from the same bow, at the same draw weight and length you will usually see the heavier arrow sport 1-3 foot pounds more of KE.
Absolutely correct. The lighter arrow will have the same KE and a much flatter trajectory. You get the best of both worlds.