Isaac:  I know what you are getting at, and I agree with you in the broader sense,  but there are other considerations to look at.  Efficient may not be the best word to use.  Maybe a better way would be to say a 30-06 will shoot a 150 gr bullet faster than a 270 winchester using the same bullet and that would be true in this example, and it's probably going to use more powder or different types more suitable for the increased dia.   I probably wouldn't use the 264 vs 300 win mag,  the 300 win is a longer case with more volume  it's really not a fair comparison.  Even if the cases were almost identical using the same parent brass, neck lengths, shoulder length, taper, etc,  Anytime you open a bottle neck cartridge up to a larger dia,  given mid range bullets weights the larger dia will almost always shoot faster.  I'm not talking comparing  a 243 case vs a 358 win that's a bad example,  but cases that are somewhat closer together in dia.   I didn't use powders as an example because some cartridges can have  wide acceptable charge weight  variations when comparing.  ÂÂ
Heavy crimps do in fact change and can increase  velocities in certain circumstances.  About the only time you need a heavy crimp is on heavy bullet revolver loads to keep the bullet from backing out or heavy large bore calibers using very heavy bullets.  Military uses them but they are multi-purpose rounds to be used in bolt actions to machine guns. The only time I crimp is on  223's, 30-30's with cannelures,  heavy revolver loads and the 458 win mag. Â
Most of the time the 7mm rem mag from the factory is downloaded under what it's actually capable of.  The freebore on Weatherby rifles is to allow running pretty much full throttle and still keeping within acceptable pressures.  Factory WBY uses powder blends  not available to us for this reason so they can keep the velocities just a little above.  It does work and not hype. Â
I would say overall your best accuracy is close to the lands, Â BUT not in all cases. Â The factories don't do it for liability issues and variations in how the rifle's chamber is actually cut. Â It definately increases pressures when you jam into the rifling. Â Barnes recommends .050 off the lands as a starting point. Â Berger recommends 0, .040, .080 and .120 off the lands to find a sweet spot. Â Every rifle bullet combo shoots different so the idea of a very specific seating depth is kind of hit and miss. Â This is something when handloading you just need to test and find out just how far it needs to be, and that's bullet specific, once you change bullets all bets are off. Â There are a certain few calibers where seating depths are so well tested the 308 for example using 168's it's almost hard to beat, Â it's possible but awfully hard. Â