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.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
