The following is a comment by ex-Les Paul Forum member 'Lewguitar' from a thread on the Les Paul Forum regarding this mod and diagram:
Here's why this "mod" works:
In diagram #1 (stock) the tone cap is connected to the pickup itself before the output of the pickup passes through the volume pot.
This means that the treble loss associated with the tone control is taken from the FULL OUTPUT of the pickup.
In diagram #2 the tone cap is connected to the output of the volume pot and the treble loss associated with the tone control is taken AFTER THE VOLUME POT has altered the output of the pickup.
So if we pick some arbitrary (and incorrect!) numbers just to paint a picture:
Before the volume control the pickup is at 100% output
After the volume control the pickup is at 90% output.
The tone control, even when on "10", reduces the treble by 5%.
So -5% of 90% output is less than -5% of 100% output.
That's one reason this mod works and seems to retain the treble.