I've seen this debate many times. Fuel mileage comes down to driver control and being smooth. I've ran both % never really seen big different in setting or mileage
Heres another "if" it was that big of a deal the pro's would all run 20% most run 25 or 30%. They are always looking for advantage. H*ll then go to 10% or less nitro.
Onroad guys less nitro to tame the engine down to be smoother not to have that power hit like a switch being turned on so it don't break traction "spin tires"