This all sounds a little odd.
To state that the fuel burn is 5% more than than Airbus promised is unlikely,the A310-300 has been flying for some year's now, so it's not as if it's a brand new plane and 'Airbus claim the fuel burn will be XYZ based on computer models'.
The 777 is not a 'charter airline'plane,it's way too expensive to operate for charter airlines,given their RPK and risk's involved.
The one and only charter operator(Air Europe), is looking to get rid of their 2 777-200's and replace them with A330-200's.
Just my 2 cents