Don't know about the 760, but the 800 is a fuel hog for a 2 stroke, 2 cylinder ski. In fact, about the only complaint I've heard about it is how much fuel it burns. I know at WOT it uses around 13 GPH plus. At cruising speed, I'm guessing you would get around 6-7 GPH.
If you want fuel efficiency you have to get a 4 stroke with fuel injection. The Yamaha VX is probably the most fuel efficient ski on the market, but would not be as fast as the GP800. The VX will burn around 10 GPH at WOT and probably cruise at around 3-4 GPH. Other Yamaha 4 strokes like the FX HO would probably be about equal in fuel consumption to the GP800, but are much heavier, bigger skis.