AFAIK, there is nothing inherently good or bad about mashing at any specific ratios. There's nothing magical about the ratio. However I do think the SPARGE VOLUME has something to do with efficiency. Not the mash ratio, but the volume after mashing that you then have left to sparge with to reach your intended pre-boil volume. Volumes matter. But not the ratio.
For high gravity beers (more than about 1.080), I use thicker mash 0.8-1.0 qt/lb, bigger sparge, and longer boil, to maximize efficiency. Big sparge, big volume, better efficiency. This just ensures a very good rinse of sugars out of the grains, that's the reason I do it. Less rinsing could be leaving behind precious sugars. I mean.... if you were to mash 20 lbs grains at a ratio of 3 qt/lb... you might not be happy with the 13 gallons you then have to boil after that, and that's without even sparging, so you'll be leaving some sugars behind in the spent grains. Conversely, if you mash the same 20 lbs at 0.9 qt/lb, you'll get only about 2.5 gallons out of the first runnings (assuming batch sparge though the concept isn't super different for fly sparge), but you can then sparge with another 4 or 5 gallons which is an excellent rinse that won't leave much sugar behind (assuming a post-boil 5-gallon batch size), though you might possibly run into a high pH concern if you fly sparge.
For standard to low gravity beers, I usually use anywhere from 1.3-2.0 qt/lb, and haven't noticed any real effect on efficiency from various ratios. I just do whatever, whenever, doesn't seem to matter much. The golden amount, in my opinion, seems to be roughly 1.5 qt/lb, plus or minus. But I have used all sorts of ratios above and below that, and don't really much care about the exact ratio on any given batch.
I don't recall the exact ratio for a low gravity beer, like say for a Scottish ale of 1.038 OG or whatever, but in an instance like this, I'll typically plan to skip the sparge as it's just not needed, so I'll add the entire volume needed for pre-boil to the mash, based on boiloff rate and batch size, drain the mash and immediately begin heatup to the boil with no sparge. If that ratio is 2 qt/lb or 3 qt/lb, I don't recall, and just don't care -- whatever it is, I'm not afraid to do it, as a small gravity beer really does not need a sparge so the only critical number is the volume you get out.... and I wouldn't want to put more water in than necessary so I'd run a calc and make sure I put in just the right volume so that nothing is wasted.
THAT is efficiency: Calculating things out so that you never use too much grain, not enough grain, too much water, not enough water..... for those who care about efficiency. After 20 years, I still haven't decided if I care or not. Meh. As long as everything is predictable and reasonably within expected parameters, I'm pretty happy. Which is, like, 95% of the time.
Yeah, so.......... I think mash ratio is just about the most meaningless useless thing to be concerned about..... within reason anyway. FWIW, I feel the same about mash temperature, and have my doubts about mash pH as well........... the list goes on............