Might as well throw some numbers out there, if we're talking efficiency along with personal opinions.
After all, this posting was about KILLING POWER, wasn't it????
.243 & 7/08 efficiency vs .338 Fed efficiency (of course, all are based on the SAME case and would have nearly identical volumes):
Muzzle energy 200 yds 300 yds
.243 100gr 1945 1332 1089
7/08 140gr 2542 1793 1490
.338Fed 210gr 3225 2265 1880 (@400/1545 surpasses 7/08@300)
As for trajectory: the 210gr .338F falls exactly 2.5" more at 300 yds than the 140gr 7/08, when both are dead on at 200 yds.
.270 Win 140gr Hornady Superformance is rated at 3090FPS and 2968 FPE (well less than smaller .338F case)
.35 Whelen (same case as the .270W) 200gr Hornady Superformance is rated at 2910 FPS and 3760 FPE
Larger, more powerful bullets, much more frontal area and bigger holes to leak.
I know which combos seem most efficient to me. But I also know that not everyone prefers the same cars, planes, houses, rifles and chamberings. That's for darn sure!!