As to real evidence "when a 300 Wby made a definable difference"--I don't know if that's even possible as I don't think you can eliminate all the variables in killing a bull elk. To begin with, bull elk are individuals too and one wonders if they don't vary at least somewhat in temperament and disposition as humans beings do.
And that's just the starting point of all those variables which goes to the heart of the decision to shoot the biggest thing within reason that you can accurately to cover those possibilities especially if you are limited in time and opportunity.
I certainly could be wrong but have never regretted taking that tact.
