Well it's pretty simple physics you take a case that usually launches a 150-180 gr .308 bullet and make is spit out a .338 caliber 180-210 gr bullet. That means you'll get a faster drop off in velocity and energy than the smaller projectile.

Federals catalog shows the current offerings in the .338 don't offer any performance gain over a .308 after 300 yards and very little performance gain over the high energy.308 load inside 300 yards.

The .300 win magor WSM spanks it pretty good at almost all aspects.