I wouldn't use short haul metrics to judge the -8F. Long haul is what it is built for. In that mission, it is easily carrying the extra 16% cargo volume and the increased weight of the aircraft for the same or better burn than the -400ERF. This before two engine tweaks over the next couple years and a rumor of more performance from the wing after some flight control changes. Also, the airplane will only get lighter as the production process is refined.
To me, it looks very likely that the ten -8 options CX has will be firmed as Intercontinental orders this year. Yet you never know what rabbit Airbus will pull out of the hat to sell the 380.