Damned if I know what "killing efficiency" is.
They are either dead or not dead. Now they can be DRT, or DOT(Down Over There). Over There can be a few feet or a few miles, and depends mostly on placement, rather than other factors, tho, as BWalker mentioned, it may possibly also involve bullet speed and animal weight when using hear/lung placement behind the shoulder. In my experience, bang-flops with boiler room placement behind the shoulder is slightly more likely to occur with fast bullets (MV up around 3,000 or higher), than those "poking along" in the mid 2K range, tho many of those IME were also bang/flops.
IME (21) moose take awhile to tip over no matter what they are hit with, unless placement is into the CNS, or at least one front shoulder is broken down. And then not always with that shoulder shot. I have had a sphincter experience with it, and now prefer CNS on moose if I can get it.
Here is some anecdotal "evidence".
I shot my first caribou with a 308 Norma Mag 180 gr. factory load at about 200 yards, and misplaced the shot slightly forward through the shoulder, rather than behind it. DRT.
The next 23 were with a Ruger 77V .25-06, using mostly 120 grain Speer C&C hand loads to near max book loads (Speer manual #8). Them 87s?? just weren't up to it. Never did try those in the 100 gr. range, the 120s working so well (MOA). Ranges from 70 to over 500, mostly 200 to 400. All bang flops except the first one, which was dead but didn't bang-flop until the second round. First round on him went from base of neck to lodging against opposite rear-leg bone at about 150 yards. The rest were either neck shots, but mostly behind the front leg shots.
We'll skip the .260 and .338WM takes- all boiler room bang-flops. (That pecker-shot with the .260 just made him walk funny until I hit him right with the second shot....a little wind drift, range mis-estimation, and embarrassment there!)
I've taken in excess of 25 caribou with 3 different rifles in .30-06, with barrel lengths of 17, 22, and 27 inches. Mostly with the 180 gr. bullet, some premium, some C&C, some hand loads, some factory. Not all were bang-flops. Some were stand until tip-over, or traveled a few yards (less than 50). I can't tell a lick of difference in "killing efficiency" among the various factors..... but the non-bang-flops were all with 180 grain loads until this last season.
Currently I am using a 27" heavy barreled M98 using 150 grain factory loads. Started with Corelokts in 2013 (all bang-flop, boiler room placements from 300 to 450 yards, estimated. 2700fps MV?
In 2014 3 caribou and 1 moose were bang/flops using the 150 gr. Hornady SST Super-Performance factory loads (3080fps MV) at ranges from 70 or so to 300 yards(RF) in a 2 day time frame. The 300 yard caribou was neck shot, the moose (30 yards) head-shot, the 200 yard walking 'bou broadside behind the shoulder, the 70-80 one quartering toward, base of neck entry, exit just behind ribs. Wound channels started with immediate expansion at the skin, and very large exit wounds. Especially the quartering shot.
In 2015 3 caribou were taken, same load/rifle. The @ 150 yard caribou was spine-shot, DRT, of course. Several inches of back-strap on each and both sides of the wound were lost- not really as much as I expected with a bone hit like that. The 290 yard (calculated after RF the carcass) and 433 (prior to shot ranged) yard kills came several weeks later, a couple minutes apart, behind shoulder, standing almost broadside. I waited until the first went down before whacking the second. Both traveled 100 yards or so before tipping over. Wound channels were much less meat damaging than experienced in the prior year, with nickel sized entry, quarter and half-dollar size exits at rib cage using the same load. The only bone hit was a rib bone on exit on one. Perhaps Hornady changed the bullet construction? After hitting the rib, that bullet also shattered the stem of the off-side shoulder blade, resulting in a base-ball size exit on the leg. And the animal still stayed on it's feet for 30 seconds to a minute, but was obviously gimping. Can't say there was any inefficiency in killing power, tho....