So the 308 Win is a better choice for elk than the 6.5s.
This information is from my rifles (308 Win, 6.5 CM, 6.5 PRC):
View attachment 234696
My 308 Win really is not any better, nor worse, that my 6.5 CM to 300 yards. For all intents and purposes, this holds true to 500. However, once I hit 500 yards my 308 Win can't be used to hunt since that is where it hits 1800 fps (assumed minimum velocity for reliable expansion). But my "wimpy" 6.5 CM still can keep hunting out to 800 yards where it finally hits the 1800 fps threshold.
My 6.5 PRC blows my 308 Win out of the water. But my 308 Win has the edge in diameter!!! I guess where these outfitters are that additional 0.044" in diameter negates the velocity and energy superiority of the 6.5 PRC at all distances.
The outfitters have the right to specify their minimum requirements. As a consumer I have the right to take my business elsewhere if I do not agree with them.
If you can repeatedly and accurately shoot a magnum, then go for. The sad fact is most people cannot. They and their target animals would be better served if they learned to check their egos and use what they shoot well.