Did some research on fuel flows for the missing 777 last year. Basically there is no difference between fuel flows at LRC and holding speed at cruise altitudes. The endurance will be the same but the distance flown will vary. That is basically true if the altitude changes. The difference between LRC at FL350 (OPT ALT) and the lowest possible fuel flow in holding (below 20,000')for a 500,000 lbs 777-200 is 3%.
For the U-2 the high altitude drops fuel flow to small amounts.