The manual says it will perform better in high temperatures with higher octane.
It's been 100+ 20 days already this summer with the next 3 weeks forecasted for more of the same. Considering that and the guidance by Ford, I'll use the higher octane (which happens to also be ethanol free in my area).
I'm not sure why you think 75 isn't normal or average. Just because something happens in the congested and overpopulated part of the US doesn't mean it translates to the other 90% of land mass in this country. "Normal" for my region is driving 80-85 mph on any highway.
I'm not saying Ford got it right. Way too early to tell. The tank for the hybrid is pressurized and relies on not being overfilled. They overfilled intentionally to be "fair," like they always do, but that can negatively impact performance of sensors and tank performance of pressurized tanks.
Wreck'em! Traylor vote here.
Also, there's a good Red Raider salesman to work with through Pollard Lubbock. I know him through RRS. I'll send along his info if you want to order locally.