The .308 gets much of its oft vaunted “efficiency” through the use of higher MAP than the .30-06 (62KPSI vs 60KPSI).
There's really four ways you can increase efficiency:
Smaller case
Higher pressure
Shoulder that catches more powder
Longer barrel
Relative to the .30-06, the .308 improves on all four (by being short action, you get more usable barrel for the same nominal length/weight). It's not any one that's the source of the benefit.
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“Efficiency” is only important to the degree that “efficiency” is the goal.
Call me crazy, but it should be a BIG part of the goal. Otherwise we'd all be shooting .30-378 Weatherbys or .30-338 Lapuas and making fun of jwall's puny .30-06 and it's pathetic external ballistics. All you're going to see is the dog's rear, jwall! See how stupid that is?
A sensible hunter has an animal (or animals) they want to kill, knows what bullets will provide good terminal performance on that animal and at what velocities, knows what trajectory they need, and then is looking for an efficient cartridge that gets that done with the minimum rifle weight and recoil. That analysis almost always favors the factors I listed above, as well as smaller calibers with high sectional density premium bullets. Looking at those factors .30-06 pretty much is never the right answer. It is however what great grandpa carried in the trenches in France. But grandpapa hated the darn thing (an inaccurate metal butt plate .30-06 with an eternity-long lock time - horrible idea) and I don't care for it one way or the other.
