Doubling the potency of fell cleaves would be a major dps buff, since instead of doing 6x500 = 3000 potency over 6 gcds you'd be doing 3x1000 = 3000 potency over 3 gcds, and you can use the remaining 3 gcds for other stuffs, unless you attach some sort of penalty that prevents you from using anything over the next 3 gcds, something like 7.5s pacification (LOL).