Brian,
There was something written about this in the Boeing Aero Magazine several years ago.
At the moment the Boeing website appears to be down ??
The gist of the article was in reference to the B767.
By
loading the aircraft towards it's rear limit, and therefore reducing the downforce required, ie lift, the subsequent reduction in drag enabled the aircraft to depart three(?) tons heavier than would normally be possible. (Can't recall the exact figure, but it was several tons).
It was of particular interest to airlines that operated out of marginal runways!
Here's the shortcut to
Aero Magazine. You may find something there.