If you want to know the difference the 6th gear makes you'll need to limit your comparisons to 2013 - 2015 Gen 3s vs 2016 and later whenever they show up. The intake systems are radically different on 3rd Gens so comparing to earlier generations is apples to oranges and will not isolate the difference the 6th gear makes.
My gut feeling is that there will be very little difference attributable to the added gear. For one thing the difference in the top gear overdrive ratio is pretty small, and for another, you don't necessarily get better fuel mileage just by gearing higher. It depends if you are in the most efficient area of the engine's power curve more than just what the rpm is.
But people asked (mostly magazine people) so Yamaha delivered the golden gear. I doubt it will go away any time soon.