I fly sparge.
I used to not do a mash out, and got 75% efficiency reliably. My sparge temperatures never got above the mid 150's Then I read that a mash out was recommended for fly spargers, so I tried it. My efficiency increased by 10% with the mash out, and the sparge temperature increased to the mid to upper 160's.
Not surprisingly (because this is what I had read), I jumped to the conclusion that the increased sparge temperature was responsible for the efficiency increase.
I now think that the real reason for the increase in efficiency is that the mash out water was thoroughly stirred before starting the sparge, and that this stirring dissolved a much higher percentage of the sugars than would be achieved with an unstirred mash.
-a.