Not trying to change the subject, but I made a sour ale that I let the mash sit almost 96 hours, you would think the wort should have a lot of noticeable tannins from the ground husks causing astringency, but so far I cannot tast any astringency. Maybe because its sour, I don't know, but it makes me wonder if boiling doesn't have some effect on releasing or intensifying the tannins? Another thing, this sour ale was a No Boil, I mashed at 147* when I started and had ~82% efficiency straight out of the mashtun into the fermeter 4 days later. The beer is slightly sweet too (last time I tried a sample anyway), which is apparently a flaw for a sour ale. Its still fermenting very slowly beneath its pellicle.
The OP wanted to increase his efficiency and longer mashes seem to have a huge effect on my brews... just how much I would suppose is debatable.