I have noted little difference in fuel mileage based on octane changing and all else being held equal in my 2016 F150 with the 2.7. When I say that, that means 87, 89, and 91 octanes (even 93) all were E10 blends and delivered similar fuel mileage.
If I fill up with premium non-ethanol fuel, my mileage goes up around 4%.
From that, my take home is it is the ethanol content, not the octane, that drives the fuel economy difference.
I monitor the Octane Adjustment Ratio which is used in controlling the engine parameters. Remember, Ecoboost trucks recommend premium when towing heavy loads or for better performance. They are rated on 87 octane, but take advantage of higher octane fuels.
The trick when deciding to purchase Non-ethanol premium is whether it is cost effective to do so, and usually the price spread is high enough, you come out behind as the better fuel mileage isn't enough to offset the cost...