DZ, if you wanna sparge so bad go setup a three vessel system. Doesn't seem like you want to BIAB or you would have gone to biabrewer.info instead of posting a steady loop of noise here.
If there is no sparge then how do you explain the huge difference in mash efficiency, simply by mashing with all the water (high) vs holding some back and topping up after (low)?
Technically, BIAB is no sparge, but it's really splitting hairs. A sparge is a specific rinsing technique added in order to get the maximum extraction out of the grain. Because the water ratio for mashing is needed to be so low, the water sort of becomes "over saturated" and not all of the sugars can be dissolved, so you rinse the grains after with the difference in volume to make up for this. It's a workaround. Ideally, mashing would just be done in the full volume and max extraction would be attained.
By keeping the grains contained in a bag, BIAB can create the lower local mash-water ratio needed for mashing, while still allowing for enough total water to dissolve the sugars. It doesn't use sparging as a solution to the efficiency problem, but an entirely different technique altogether, which is precisely what makes it unique. No reason to chastise someone for pointing this out.