I would expect that Ford
I would expect that Ford would tune the Ecoboost for peak efficiency and power at 93 octane and engineer in a backoff range to 87. It could be tuned higher than 93 possibly in case someone wishes to add some higher octane fuel to their tank. Conversely it may back off further if bad gas is used. I would expect there to be a reasonable range of efficiency engineered in from about 85 thru 95 octane and beyond that no change in tuning aside from malfunctions.