Sticky:
http://forums.mtbr.com/wheels-tires/tire-pressure-all-around-xc-riding-561602.html It has many pageses of discussion that might be of interest.
Another Sticky under Wheels and Tires that might be of interest:
http://forums.mtbr.com/wheels-tires/tire-test-results-german-bike-magazine-419392.html
I'm kind of agnostic over the whole thing. My recorded times are better, even in technical climbs, with smaller, lighter tires run tubeless at medium low pressures than with bigger heavier tires, but I much prefer riding with bigger, heavier tires for the comfort and increased fun over all, especially in the very uneven terrain I ride. I think part of the reduction in energy expended with lower pressure has to do with a more level "axle path". The axle is not going up and over every feature as much. Also, with the wider, shorter patch, the angle of attack at which the leading edge of the tire hits the feature is steeper, so less of a head on impact. I have a NN/RR combo I ride with on my 26er. When I run the 2.25 NN front/2.1 RR rear Vs the 2.4 NN front /2.25 RR rear, my times are consistently faster on long technical climbs. Both NNs are the same versions and both RRs are the same versions.