Well the article spoke the historical truth, The original 404 was made specifically to duplicate the 450/400 and it did a good job of that. But the 416 Rigby was made to fire a bullet of like weight 200-225 FPS faster. Some hunter have noted the "shock" effect that extra velocity gave.
My experience with the 404 was mostly with the German RWS ammo which also fired a 400 grain bullet at 2350. But I had a few old Knyoch cartridges from the 30s, and to be honest, they seemed to work just fine too.

I have never killed buffalo, lion, hippo or any African game with a 416, but I have killed 2 elk with a 416 Taylor which fires the exact same 400 grain bullet as the Rigby at the same speed. (mine chronographed at 2365 with my load) Works fine. But I think I would have to live with a 416 for a few years to have enough knowledge to speak honestly or intelligently in any valid comparison.

If bullet construction were equal, I cannot see any reason I would believe a 400 grain bullet at 2350 FPS of .423" is going to be noticeably different in it's effect then a 400 grain bullet at 2350 FPS of .416" diameter------------ and if it were to be different the nod would probably go to the bullet of the slightly larger diameter. Or am I missing something here?

Some will site the fact that the Rigby (and Remington and Weatherby and probably 2-3 others) can all be loaded to higher velocities. I do not despite that, but I'd ask why could you not also do the same with the 404? If the action is strong enough to hold the additional pressure, and the thrust on the bolt face is equal (or slightly less in the case of the 404) I can't see any reason the 404 could not be loaded to higher speeds. I seriously doubt the addition of a bit more speed has any effect that is truly beneficial in killing large game unless it were possible to make a solid bullet that would not deform in ways we don't want and drive it a LOT faster. Maybe 2900 or more. But I am only guessing here. I base my statement on the words of a good friend who used the 416 Weatherby for 3 years and later told me it was never any better then his older 416 Rigby and sometimes the faster bullets acted erratic. The Rigby was not erratic.

I also see NO reason why I'd want to speed up with either a 416 bore rifle or a .423" bore rifle past about 2400.

So I think the 2 shells as loaded today are probably equal--- or so close as to make no difference at all.

Just my thoughts in passing..........