With 87 octane, from what I gather, will keep your knock sensor busy, and ignition timing pulled back. Available torque and hp is not to potential even within the factory tune. The latter I'm certain of. Whether or not boost is reduced while sensing knock I'm not certain of, but would logically assume so. This is now a premium SUV, and like the others such as BMW, MB, Audi, etc., do benefit from 93 octane. I think ford put the "87 octane ok" caveat in the manual to ease the minds of folks not willing to drop that extra 10 spot at every fill up.
Curiously, my manual omits the fuel requirements print under the 3.0 tt engine section.
While cruising the various EcoBoost forums, I found this interesting post from a fella with real world results on his sho with various octane gas. Plausible?
"No opinion as such but here is the real data from the spreadsheet on my SHO with EcoBoost. I did a fuel test with Regular, Mid Grade and Premium to see what the difference would be as far as the MPG. I ran 10 tanks of each blend using the same gas brand, Quik-Trip which is a Tier-1 fuel, in all tests.
Regular: 17.9MPG
Mid-Grade: 19.7MPG
Premium: 19.8MPG
As far as the Cost Per Mile, which is actually better than "MPG" only when looking at cost, the results were:
Regular: $0.20
Mid-Grade: $0.17
Premium: $0.18
Regular was the most expensive gas to use in my SHO."