as reputable as you are. i dont belive u untill i see it with my own eyes. simply for the fact that if my r33 185rwkw conservativly can only make 460/klms sydney west driving with me rolling in neutral every where i could. though hardly any of those klms were hwy. i can hardly see a car that is making 350rwkw making it that far, with as u say, "not driven easily". you were no doubt rolling everywhere i would think.
have you have changed things like gearing in the drivetrain, 02 sensors ra ra ra?
skylines are petrol whores to say the least.