Piece of p*ss for the 777-8X. Advertised range of 8,700nm with 350-375 pax. Reduce that to 300 pax and range is pushing 9,200. Couple of aux tanks get you over 9,500nm. Boeing finds a few % of improvement in weight or aerodynamics, GE gets a few % out of the engine and there's 10,000nm to deal with wind/wx. The extra weight of fuel in aux tanks won't be much of an issue at all since the -8x shares most its structure with the (bigger/heavier) -9x. Not sure but I bet the airbus is pretty close too.