Completely agreed
@WAB. Same goes for cape buffalo etc. This discussion goes back all the way to 'Pondoro' Taylor's 1948, what did it call it?, "KO factor" if memory serves, and my personal field experience, although infinitely much more modest than Taylor's LOL, absolutely supports his perspective, 'Doctori' Robertson's, and yours.
The challenge in this thread is "One rifle for North American big game," therefore, by definition, the answer will be a compromise, which, again by definition, is not ideal in ALL situations. Of course, I would take a .375 (H&H, Wby, RUM, Ruger, etc.) any day over a .300 (H&H, Win, Wby, RUM, etc.) for dangerous game, and truth be told - believe the timing of this discussion or not ;-) - I am flying tonight to Africa with a cape buff on license, and my caliber of choice is not even the .375 H&H but the .416. Rigby. Now, how is that, for TOTALLY agreeing with you that "they are much more impressed with size than speed" LOL?
But I think that neither you or I would recommend the .375 or .416 as "One rifle for North American big game" (although truth be said, if you were willing to limit yourself to 200 yd, the good old .375 H&H would do quite well on pronghorn, as it has done for a century on everything that walks in Africa, as THE "one rifle safari" gun...). So, for lack of a .375 with 300 gr premium bullet as "One rifle for North American big game" I will still, and I suspect you will to, recommend a .300 with 200 gr premium bullet, or .338 with 250 gr premium bullet, over a 7x57 for brown bear, right?