Well, I think a mash out and sparging with water to keep it just below 170F helps a lot, at least it has in my experience.
Sure, a 170°F "mashout" can improve efficiency if you haven't reached 100% conversion, but it doesn't seem to matter if you've already achieved 100% conversion at the end of the sparge.
I have pretty much adopted a mash temperature sparge routine and get around the maximum lauter efficiency mathematically possible for the volumes that I use. I get the same for a 170°F sparge. I can't really speak to very high gravity beers, where you might expect to see more effect on viscosity, but for normal gravity beers any effect of high temperature is apparently on pushing saccharification to completion.
Isn't that a circular definition? You get better efficiency because there's better conversion? What other kind of efficiency is there?
The difference is in getting higher efficiency by improving conversion or by sparging more stringently, which risks adverse effects on flavor.