Most of my manuals show that beating published '06 factory loads isn't a trick at all.
The '06 is only "inefficient" as it relates to 150 and 165 grain loadings. The .308 can come close, using around 10-grains less powder. But pump up the volume and it's as they say, "No replacement for displacement." There are no great mysteries here; to get more speed, we need to burn more powder, to burn more powder, we need more cartridge capacity.
As a bonus, I've noticed that handloading for the '06 is relatively easy. I haven't met one yet that was super picky. Can't say the same for the .260 I had; it could shoot well, but you had to remember to be careful and feed it only precise loads.
But the fact that the cartridge was adopted as the U.S. Rifle Cal. 30 of 1906, is what makes it an '06. The case also serves as a rough base for the .25-06, .270 Winchester, .280 Remington, .338-06 and .35 Whelen, among others. Oh, almost forgot the wildcats 6mm-06 and 6.5mm-06.
Of course, the .308 Winchester served as the base for the .243 Win, .260 Remington, 7mm-08 Remington, .358 Winchester and now the .338 Federal.