purely out of curiosity, if I use StackSizeMultiplier with this, and, say, make thousands of chests have one stack of 2000 instead of 20 stacks of 100, will it improve things? and will there be any difference at all compared to locking off those slots in the chests manually?

In theory, I think the answer will be yes.

I actually made after a discussion about locked slots. It was explained to me that factorio will essentially search from the first slot to the last to find an empty spot when inserting. And factorio searches back-to-front when taking items out of chests, even when the slots are locked. (This is how inserters can take items already placed in locked slots out of the chest's inventory.)

So if you set inventory size to one, it should make the "take out of chest" operation equal to the "insert into chest" operation in terms of search time. Factorio is really well optimized already, so you would need to be using a large amount of chests to see the benefit of this.

Do let me know if you or someone else happens to perform any tests on this, as I am interested to see the results.
Cheers! :)