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Fuel octane is a science within itself.
Going higher than 87 adds the ability to change engine timing curve increasing torque and efficiency. Octane number is resistance to detonation, higher is more resistant. Computer controlled engine that use various inputs such as knock sensors to control mixture/timing can benefit from higher octane under certain conditions. Essentially going better octane on fuel with likely give no negative effect except increased cost. Running down road lightly loaded in lighter units would probably see no benefit. Heavily loaded, towing, hot weather may see a benefit from higher octane.
I race chevy small block drag cars with fixed fuel/timing. I start out running pure 110 octane leaded race gas at $10.00 or more/gallon with new engine combinations. Then find where engine works best and start cutting the race gas with premium pump gas. I have to pull plugs and look into chambers for signs of detonation as I water the race gas down. I do this strictly to reduce operating cost. When I reach what I feel a safe I will stay at that current mix. My current setup that won NoBox/Mod class at Super Chevy Show last month at Palm Beach is using 40% race gas and 60% pump gas at 12 -1 compression ratio. I have run and won events at 14-1 on pure pump gas with reduced timing. Race gas does not inherently make car faster by itself it just allows more tuning latitude. Running pump gas with 10% ethanol does require a small increase in jet size. This is why many computer controlled vehicles saw a reduction in MPG when ethanol became mainstream. Towing with my 2007 Chevy Suburban I always used mid-grade or premium as a safety measure to minimize pinging. On more than one occasion MPG improved with addition of premium when not towing.
I did test the RV with full tank, 49 out of 55 gallons of premium and saw no net change in MPG which tells me the unit power/gearing is pretty effectively matched.
Going to better, higher octane fuel is a reasonable choice. Ford may have decided to tap any detonation reduction/efficiency improvement in heavier chassis by adding that statement about 91 octane. Plus I can see where having an extra safety margin is never bad.
This is all based on what I have seen and experienced over the last 15 years racing and towing drag car all over southeast.
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