When an owner's manual states that 91 octance is required, then that's pretty much it. Engines like this do not tolerate lower octane fuel well. Over time, damage can result that will be far more expensive than the fuel cost saved by using a lower grade.
There are cars that merely recommend 91 octane. Cars like this have the ability to run on a range of octanes (a minium is usually specified regular 87), but the horsepower and 0-60 advertising claims were made using 91 (or in some cases, 93). So they recommend that fuel for best performance. The new Cadillac CTS is such a car.
Ethanol doesn't enter the picture unless you have a flex-fuel vehicle, which you don't. A lot of specific mechanical and engine mapping software considerations go into a flex-fuel engine.