Yes, ethanol is a high octane blending component but octane is expensive so most refiners make a subgrade gasoline that is blended up to the posted octane with ethanol. So the blend you buy should be the same octane as posted on the pump. My experience is that some premium fuels seem to knock more than others even though they have the same posted octane rating. I suspect this is because octane rating at the pump is actually an average of two methods: research and motor. I suspect that the motor octane (the lower of the two) is higher for the premium fuels that seem to knock less. Ethanol can be hard on some of the older elastomers on cars that were not designed for it.

Dean