One thing about heavy arrows that I have experienced... You can go too heavy with arrows, exceed the bow's efficiency range, and slow the speed down to the point where KE does begin to drop off. Unless you've done that, you will lose some KE going to substantially lighter arrows.

I don't really like going heavier than 10 grains per pound of draw weight. I have gone up to 12 grains per pound with a 74" longbow. With my draw length, I also don't like going lighter than 7 grains per pound.

Just to throw another monkey wrench into the works... Anyone ever thought about how short draw lengths vs long draw lengths impact bow efficiency with various arrow weights? I don't have any evidence to support my opinion on this, but I would bet that short draw lengths would be more efficient with lighter arrows than they would be with heavier arrows. Long draw lenghts would lose efficiency with light arrows. And we could also look at efficiency with short vs long limb length at various arrow weights.