91 Grade - no ethanol
Ultra 94 - with ethanol
Notice the difference in RON number from 91 grade without ethanol (97.2) and the RON of Ultra 94 (101.5). The difference is 4.3 numbers. That is significantly more than the 3 numbers difference between 94 and 91 AKI labelled on the pump. That is why gasoline with ethanol can be the best performance fuel for today's cars.
That's comparing apples to oranges. I want to see straight 91 grade without ethanol compared to the same gasoline now with 10% ethanol added. I agree that the RON is going to increase but with 10% it's going to be slight. Matter of fact I don't even think the R+M/2 number on the pump even takes into account the added ethanol especially since it's added after the fact. Further, the intention for adding ethanol is not to boost octane, but to tell the public we have reduced our gasoline consumption by 10% with something produced in this country. Unfortunately in reality it doesn't work that way because of poorer mileage.
Here is what Ford has to say about E85 and remember that this is 85% ethanol and only 15% gasoline:
Want to add power to Ford's all-new 5.0-liter V-8 for the 2011 F-150 without making a single hardware or software change? Just burn E85 ethanol fuel.
360 horsepower (at 5,500 rpm) and 380 pounds-feet of torque (at 4,250 rpm) running on regular unleaded gasoline.
Burning E85 fuel boosts 5.0 engine power to 375 hp and 390 pounds-feet of torque, said Mike Harrison, Ford's program manager for V-8 engines.
Ethanol has a higher octane and heat-of-vaporization point than gasoline, meaning it combusts at a higher temperature and with greater force (higher compression) than gasoline, while also having a greater capacity to cool the fuel/air mix in the cylinder before combustion.
This inherent efficiency is what enables the 5.0 to produce more power while burning E85 instead of regular unleaded fuel.
There's a trade-off, though. Even though E85 combusts with greater force, it has less energy per gallon than regular unleaded gasoline, so fuel economy is worse when burning E85.
There's the reason straight ethanol is used as a racing fuel but at 10% in an engine designed for gasoline the benefit is less than zero.