I surveyed (bird towing) the top of the Chilean Andes from Copiapo Chile to North of Antofagasta from West to East to the border of Argentina and back everyday for almost 4 months (time off in between) Did that with a B205 -17 and regular blades (single hyd) the everage fuel burn at close to sea level is about 610 to 615 lbs an hour, Le whole job was done at between 20,000 to 25,000ft DA and the average fuel burn at that altitude was about 400 to 415 lbs an hrs. I know the maximum altitude 204 blades will go to, but the T53-17 would have had no problems going higher.
JD