Let's correct a few incorrect assumptions in some of these answers.
Rated power is set based on a industry standard fuel that is equivalent to ~92 Octane, not 91. Notice I said fuel, because it is not gasoline. It is used by most manufacturers now so there is a standard. It is used for MPG, and Power ratings stated by the manufacturer.
Will the EcoBoost make more power with Stock tuning on 93 vs 91, YES! One more time for those in the back. On Stock tuning the EcoBoost motors in the 15+ Expeditions will make more power on 93 vs 91. You could even argue that they will make more power on 95 Octane vs 93, but the returns start to diminish quickly because the stock mapping just doesn't give enough, because the tuning is based on Torque demanded. That more power is going to be even more noticeable under heavy load or high temps, because of the additional Octane it will pull LESS timing as the intake charge temp increases, because there will be less knock.
Normal timing adjustments due to knock being sensed due to Octane changes from 93 to 87, WILL NOT cause a check engine light. The motor simply detunes/pulls timing, and demands less power. Also to be clear, the motor does not tune up to 93, it detunes to 87. It is important to understand that difference. It wants the higher, and settles for the lower, not the other way around.
If you got a Cat Error, with no other errors, it might actually be the CAT's. If you got it with a misfire or other similar code, maybe just a bad tank of gas? Like others said, a real scantool could shed light on that based on stored codes.
