Efficiency results - gravity of final runnings

For my recent brews I have achieved pretty decent results, with efficiencies around 84%. One interesting observation is that my 3rd and final runnings (the last dribbles from the 2nd batch sparge) are still pretty high, at about 1.027.

This would imply that if I did a third sparge, or alternatively used much higher volumes for the first two sparges, that I would get even more sugars, and hence higher efficiency.

I did some extrapolations in excel and it seems that a 3rd sparge would result in a gravity of 1.014, and would extract an additional 7% of sugars.

Following is some data from my last brew. The data in the last column (4th runnings) is extrapolated.

1st runnings 2nd runnings 3rd runnings 4th runnings
Gallons 4 8 12 16
Gravity 1.093 1.052 1.028 1.014
Sugars 372 208 112 56
Total sugars 372 580 692 748

So one question:
Could these same amount of sugar points (748) be extracted using 2 sparges (3 runnings) but using the same amount of water (16 gallons). I'm sure that there is some loss of efficiency using fewer sparges of X gallons of water, versus more sparges with the same X gallons. Any ideas what this efficiency change would be?

Second, if so, this would indicate that I would be getting efficiencies of around 90%. This is a bit hard to believe, considering that just recently I was achieving effic's in the mid 70's, with the same equipment.

You have to draw the line somewhere. If you sparge more volume, you'll have to boil longer. It sounds like this was a high OG beer and you have to expect a slightly lower efficiency due to how much wort stays in the grain.

Not exactly. I was shooting for 1.057 and ended up with 1.066, with efficiency in the low 80's.

In this case if I did the 3rd sparge and extracted that other 7% sugars, and then boiled and boiled and boiled, I'd have ended up with way to high of a gravity.

So I want to scale back the water and grain proportionately to get the desired gravity at the desired volume without excessive boil-down required. But as I say, the resulting efficiency would be close to 90%.

I guess a way to state the observation, which is admittedly not terribly profound, is that higher efficiency is partly a function of using relatively more water to rinse fewer grains. I guess its just been interesting for me as a noobie to be seeing pretty high efficiencies, and observing that I'm still leaving some on the table, or in the MLT as it were.

Are the numbers in the OP reasonable if I have an appropriate amount of grain and make sure that I would collect the same amount of wort per running? I've stepped up to doing ten gallon batches lately, and I would like to make a big beer, but don't want my efficiency to drop as much as expected with making a big beer, so I'm debating something like a single mash and then adjust running so that I end up with about 7 gallons of high gravity wort in one kettle, and 7 gallons of lower gravity (collecting this will up my efficiency), into another kettle. Then do two boils, one to make a big beer (maybe a RIS, or IIAA ir IIPA), and one to make some more sessionable. It would probably be something like collecting the first runnings and some of the second runnings into one kettle, then the rest of the second runnings and all of the third runnings in my other kettle.