I believe the new 6.5 Wby is based on the 270/257 Wby casing, not the .300 Wby so it's essentially a .264 Win Mag with a double radius shoulder.
The 6.5-.300 Wby is a wildcat that is close to the 26 Nosler just with the double radius shoulder.
The 257 Wby and the 6.5-300 are both at the top of the "efficiency" scale and require 26"+ barrels, but both are pretty awesome rounds.
We have a line based off the .300 Win Mag and I think I'm going to build a .257 Diablo next year for antelope/deer.