You may actually be getting a slightly higher gravity when doing a mash-out. The reason is that you will gelatinize more starch, which may then be converted to sugar, as you raise the temperature. You do not start gelatinizing the smaller starch particles in barley, which make up about 10% of the total, until you reach 70 degrees C/158 F, and you will not gelatinize all the starch until you reach 94C/201F (source is Kai Troester). And as alpha is still active at mash-out temperature there ought to be an effect. It is probably small, though. And it will be totally meaningless anyhow, because it will be mostly dextrins, and those will have no effect on any of the qualities of the beer.
I don't think any beta will survive into the mash-out. It is denatured at temperatures above 65C/150F, and though it doesn't happen instantly, it won't last long above that limit. It does work faster when it gets warmer, though, so a single infusion mash at 67C/154F will still get you a very fermentable wort, not much lower than at 65/150, which is the ideal (Source is table in Palmer's How to Brew
.)It is also generally less hardy than alpha.
I never do a mash-out, but I do normally use a long step at 72C/162F, which according to Troester (who refers to Narziss, if I remember correctly. Or was it Kunze
?) produces glycoproteins, which give you a better mouthfeel, and better foam stability. I do think it works. The effect isn't dramatic, but still worth the effort, I think. And so that is perhaps an alternative to the old mash-out? (I think it gives a couple of gravity points, but those are, as I said earlier, meaningless points.)
As for the effect of the lower viscosity, I can't say. I've sparged with water at 30C/87F, and couldn't notice any diminshed efficiency, so my guess is that the effect is minmal at best.