Well, I don't know much about energies, but I do know that any 20 gauge sabot slug I've ever seen will drop a deer out to 125 yards or further (if the shot placement is good). My best friend has killed several deer with his 20 gauge at 120+ yards - no tracking required for any of them. I'd say that the trajectory & shot placement are the most important thing- if you put the bullet where it needs to be, the deer is going down.

I personally don't find bullet energy discussions useful for shotgun slugs and muzzleloaders because they generally kill by direct tissue damage, as opposed to centerfire rifles which tend to kill more by shock. A heavy, slow, shotgun slug or muzzleloader bullet will usually kill a deer quickly if it is placed properly, regardless of what the "paper" energy level is. At least that's been my experience. My hunting buddy's daughter has taken several deer with .410 slugs - which are (IMO) very underpowered for deer, but the range was short and the placement was good, so the deer went down.