Think of a typical "Hill style" ASL. The grip is 1.5" to 2" deep with the limbs on a plane with the back of the bow. With the forward handle, the plane of the limbs is at the belly. So, given a two inch deep "Hill style" grip, compared to a forward handle, the forward handle limbs are being drawn two inches less.
So, this next may or may not be tested but here's what I'm thinkin.
Given bows of each style that have equal draw weights when drawn by the same archer to the same anchor point, the H style will pe propelling the arrow through a power stroke that is two inches longer. Both bows accelerate the arrow continuously from departure from anchor point to the nock leaving the string. The forward handle will be accelerating the arrow two inches less and therefore with less speed (efficiency).
Could be stinkin thinkin but interesting to think about and discuss.