I recall a presentation from a QF fellow at the RAeS some years ago .. relating to a study (which was driven by a pilot's interest, rather than the engineering folk at it turned out) which looked at rescheduling the tailtank usage on the 744 to constrain the aft cg a little more ... long range it was worth a couple of (fare-paying) extra passengers .. over a year, something in excess of a lot of money.