I'm in slight disagreement with the reasons for mash-out that are commonly given by home brewers. Although it matters little, but mash-out doesn't denature all enzymes. Its temperature is actually chosen such that the alpha amylase is not completely denatured. It is still needed to convert rouge starch that is released during lautering. Not so much a worry of home brewers, but bigger brewers do seem to worry about that. But mash-out does fix the fermentability by denaturing the beta amylase. While a-amylase can create fermentable sugars, it is not very good at doing that and the fermentability increase during mash-out is quite low. The benefits of the mash-out are increased protein coagulation which increased the size of the flocks and the lowering of the wort viscosity. Both help the run-off speed which is especially important in fly sparging.
If your sacc rest didn't convert all the starches, you may see an improvement of the efficiency when doing a mash-out. This is because the a-amylase becomes more active at the higher temp and is able to convert some more starches more quickly. But most of that efficiency gain will be unfermentable dextrins so you should not rely on a mash out for good efficiency. Just today I saw a jump from 90% to 95% conversion efficiency through the mash-out. After lautering this translated into a 3-4% efficiency boost.
ajf, I'm surprised to read that your efficiency dropped with a mash-out. Was that for the same recipe?
Kai