From the 10 or so long haul trips I've done HKG JFK and YYZ I would agree with the above posts. Going faster than .84 seems to burn a lot of gas but only save maybe 15 mins overall. Slowing down also doesn't seem to make any difference either.
On the 200's and 300's ( non ER ) we are told CI 20 is supposed to give the lowest fuel burn?? but the reality is that on short sectors of 3 hrs or less it makes no difference at all.