Just a couple of clarifications...
- octane rating has nothing to do with how much energy (BTUs) is available, all grades of pure gasoline have exactly the same energy content. Likewise, all grades of E10 have exactly the same energy content although E10 has measurably less energy than pure gas (about 3.5%). You will get both more power and better mileage using pure gas than any ethanol mix.
- the only difference between grades of gas with different octane ratings is the resistance to pre-ignition and detonation (engine knock). In unbranded gas (Hess, RaceTrac, Circle K, etc.), there is no other difference and there is absolutely no reason to buy a higher octane grade than your engine is designed for. Branded gas (Shell, Exxon, BP, etc.) often has more additives in their "premium" grade than in their other gas. It will produce no more power or mileage (unless your engine is designed for it) but it might keep the fuel system a little cleaner.
- higher octane by itself will not produce more power. However, higher octane is more resistant to engine knock so using it allows some engine designs to increase their ignition timing advance thus producing more power. All modern engines have knock sensors that allow them to adjust timing based on measured engine knock. Most are designed only to retard the timing to protect the engine if a lower than expected octane fuel is used. Some, like a few Ford engines, are designed to increase the ignition timing to take advantage of higher octane fuel. The EcoBoost engine has this adaptive timing. The 3.5L V6 does not. Whether the 3.7L has it is debatable - Ford does not say it does but dyno tests on Mustangs with that engine show a few horsepower gain using 93 octane compared with using 87 octane. Even so, the difference is not enough to justify the extra cost - 30 cents per gallon (about 8 percent) higher cost to get about 2.2% more horsepower... not enough to even feel on the seat-of-the-pants dyno.