It is hard to say why the performance of your wifes uncle's 416 is the way it is on elk without knowing more details, but when you look at professional guides like Jim Shockey that carry a 416 as his backup rifle when guiding things like grizzly, it makes you wonder if it isn't because of personal experiences that are favorable. Big bullets like a 416 that penetrate too quickly and exit, might not be the best bullet for the type of game being hunted. On the other hand when a compromise between a solid and an expanding bullet is needed on large dangerous game, the bullet might perform better. My personal experience is with medium calibers at high velocity and they have proven themselves as devasting on game. I am over 95% DRT with both my 338 ultra and my 300 ultra and that is more than I can say with any of the multiple animals I have harvested with both a 30-06 and 7mm mag.
My guess on that and bigs trouble with the 45/70 on deer is this, not enough resistance to expand the bullet, I used a 350 rem. mag occasionaly for about 10 years, never knocked a deer down with it, and always had to track them, and not much expansion either. Once a bullet exits the remaining energy is basicly wasted.