Oh - reading comprehension failure. So he’s adding a bunch of hot water at the end? I focused on the “no-sparge” part and figured all of the water was already there. So it’s really the addition of hot water and essentially creating a step mash that you’re talking about as the cause, not mashing for 90 minutes instead of 60. That makes more sense.
Right, 90min at 150F and then adding my “sparge” water to step up to 158F or so for a short period. This is what I’ve been told was typical of no-sparge, and the bump in efficiency puts me very near what I get with a single infusion at 150F and a batch sparge. What I haven’t tried yet is a no-sparge with water that doesn’t give me an alpha step. Thats on my list since I’m brewing some dry beers and have gotten the upper end of style FGs a couple of times.
You could do no-sparge with water that doesn’t give you an alpha step, but why not just mash with all of the water to begin with? There’s obviously space in the tun.
I generally stay between 1.5 and 2qt/lb, thats a nice consisntency for a mash. I’ve heard of people (commercial operations mostly) mashing up to 3qt/lb, so I suppose I could. I don’t see a real benefit to it, other than only warming up water once. I was thinking the step mash aspect would be nice but I just wasn’t expecting as pronounced an effect as I saw.
I’s all about matching the recipe to your procedure. If you like the bump in efficiency, you can adjust the procedure and recipe to get the fermentability you want. Maybe mash a little lower for the first step, add a little sugar to dry things out, whatever you like. Or not. 
Yes, mashing lower at the first step is likely key to get your attenuation up.
Kai
I used to add a little sugar to most recipes but I quit that in search of more malt flavor. I may try mashing lower but starting at 150 and having it dropdown a couple of degrees during 90min, seems like a pretty attenuative way to go. At least I was getting FGs around 1.010 with this method, before I tried the no-sparge step mash and/or mashout.
I think you’ll need to check if this FG is really from mashing or if the yeast is not going all the way. The fast ferment test will show you that.
I can easily get attenuation levels in the low 80s by mashing at 145 F for 30-45 min and then mashing at 160 F for another 30-45 min. Conversion is also complete (close to 100% conversion efficiency) after the 160 F rest.
Kai