This is just an assumption here, but I'd bet that the guy who told you he is shooting elk at 500 yards with a .257 wby probably wouldn't know what 500 yards looked like if you stood him on the 500 yard line of a rifle range.

With a bullet that is made right, and a shot that is made right, I have little doubt that it would do the trick. But elk are a lot tougher than deer are, and at ranges that extreme in the sort of country elk tend to inhabit.... nah... I wouldn't do it. I think there are much much much better choices, espeically for shooting elk at that kinda range.

If you are hunting in the timber and your shots are inside 150 yards or so... I doubt it matters much. The sweedes shoot the hell out of moose with those 6.5x55s. And there has been plenty of African game taken with 7x57s and .30/06s that are as large and tough as elk. I just really think there are better choices for you though.