That extra lethality (that is to say, very quick kills) isn't there with "harder" .25 caliber bullets, as I can personally attest from my experience with monolithics in the .257 Weatherby. With cup-and-cores it's there, and to a major extent with "partial cup-and-cores" like Partitions. The .375 H&H doesn't destroy nearly as much vital tissue with typical bullets, both because of the lower muzzle velocity, and because most .375 bullets are built to retain more weight.
My experience with quite a few different bullets is that quicker big-game kills result with more bullet weight-loss, which destroys more of the internal organs, whether the weight-loss of the bullet is a result of its construction or increased velocity. More than one bullet company has come to the same conclusion, including a major European ammunition firm, which shot over 500 animals during development of loads that would drop animals quicker, so they wouldn't make it across the border of neighboring land. This is a big deal over there, because the landowner owns the wild animals, and a deer that drops on a neighbor's land legally belongs to the neighbor.
John, you've touched on the very thing that makes most "terminal ballistics" discussions devolve so quickly on the interwebs. What most hunters are really more interested in is the terminal effects of their bullets, and terminal ballistics is just a subset of that field of study. I have noted for years that students of terminal ballistics often miss the more crucial points of Fackler's academic papers, which had far more import in the understanding terminal effects than of terminal ballistics.
Terminal effects must take into account ballistics variables, which can be neatly summarized/symbolized in mathematical physics equations(although if you've read Duncan McPherson's book, you'll quickly discard any notion of it being "simple" physics!!!). But Terminal effects must also take into account the anatomy and physiology of the target animal, the path of the missile into/through the body, and the behavior of the bullet within the target animal's body.
A bullet's tendency to fragment (= weight loss) inside the target animal has a direct bearing on a shot's potential lethality, as you point out. Vincent DiMaio's opus on Gunshot Wounds has several photos of the "lead snowstorm" effect of highly frangible bullets in the human chest, and while he as a pathologist doesn't comment on the rapid incapacitation potential of such GSW's, as an emergency physician I will attest to the extremely short time-frame of survival of such injuries. It's this effect that manufacturers such as Berger and JLK have tried to improve upon, and in my estimation they have been very successful.
You are spot on. As to the fragmentation creating quicker kills it all depends on where the fragmentation takes place.
I got banned on another web site for a debate that happened on this site. That's a first
