With an Unchained opener, I'd only be able to get in one Fell Cleave though. 2 if I do the pre pull shenanigans which I won't be doing.

The difference in potency over Berserk's duration (adjusted with buffs added would be ~5593 vs ~4326). I didn't use Fracture in the math, but in practice I would use it as OT). Would a Drk gain 1200 potency over the first 20 seconds, starting a couple GCDs in, as OT?