I think people get held up on "efficiency", when really what they're trying for is "quality". Efficiency is basically the inverse of "losses"
If you managed to get 100% of the available sugars in your malt into your fermenter, then you would have 100% brewhouse efficiency. This is not feasible.
What is feasible is to reduce all your losses to the minimum, and then you will have maximized your efficiency.
The problem is, something known as "Lauter Efficiency", which is how efficiently you remove the extracted sugars from the mash. If you over-lauter, well, actually over-sparge, then you can end up with quality problems.
So, to maximize efficiency and quality, don't over-sparge... simple really.
To expound
Conversion Efficiency * Lauter Efficiency = Mash Efficiency
I regularly achieve 99% conversion efficiency, achieving a high conversion efficiency does not affect the quality of the brew. Achieving it is basically about getting a good grind, and a good mashing process which allows you to achieve close to the same results which would've been achieved in a congress mash.
You measure it by measuring the gravity in your mash tun. The problem is you need to know how much liquid (including dissolved extract) is in your mash tun... which is tricky, but doable.
After you have managed to achieve 99% of the specified 100% extract, you then have the lauter stage... where we try to separate the sweet wort from the grains. The first runnings are easy... but the next thing is we generally try to rinse the remaining sugars out of the grain... this is where quality problems can occur. If you over-rinse you will extract tannins and other undesirables from the grain.
People talk about reducing their efficiency in order to achieve a higher quality. What they should be talking about is reducing their lauter efficiency in specific.
Any deadspace losses in your mashtun are just pure losses. They don't improve quality, they just waste resources. Try to reduce them through good design and process!
After the lauter, we end up with all the sugars in our kettle, and we've suffered losses to the mash tun and lauter. This efficiency, the "Into Kettle" or "Start of Boil" or "Mash Efficiency" is what is really key, if you are pursuing quality.
After the boil you have another transfer, to the fermenter, and as a result of that transfer you will leave trub behind in your kettle, the hot and perhaps cold break, maybe losses to hop absorption etc etc. Minimizing these losses will increase your overall efficiency without affecting quality. Perhaps a good false bottom in your kettle would help? Increasing these losses in a mistaken belief that a lower brewhouse efficiency would result in increased quality is misguided. Just pour some wort on the ground, it will have the same effect on quality.
No one ever talks about Into Packaging efficiency... After all, the trub in your fermenter doesn't affect the quality of your beer. Just like the trub in your kettle doesn't.
The only parameters which affect the quality are the Mash Efficiency parameters. Conversion Efficiency of 95+% is easily attainable, and a lauter efficiency of about 80-90% is easily attainable too, without affecting the quality of your mash.
My point, which is probably lost in that ramble is that aiming for a certain Into Fermenter efficiency in the hopes of ending up with a "quality" beer is not the way to do it. The way to get a quality beer is to aim for complete and efficient conversion in the mash, and then to get as much of those sugars into your kettle as you can,
without over-sparging.
Of course, the easiest way to not over-sparge is to not sparge at all, and some people think that any sparge is too much
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