I'm experimenting E85 as a way to boost 91 AKI CA gas to 92-93 AKI. Just one gallon to raise CA Premium 5.7% ethanol to 10-11% ethanol. N54 is designed to handle at least E10. Some guys here have claimed that the Brazilian market cars are the same and the minimum ethanol there is E25.
No problems at all running E10 mix (as expected), and the car definitely runs better. Impact on fuel economy is minimal.
It's difficult to put an exact number on octane gains with ethanol. I think it's different between naturally aspirated and forced induction cars. FI cars gain more anti-knock from the ethanol due to charge air cooling effects. I can say based on a LOT of research that boosting 5.7% ethanol to 11% will raise the AKI at least 1 point.
However, diminishing returns start at 10% ethanol, so to get another point AKI would probably require 22% ethanol. This effect increases with the ethanol percentage.
In CA where 100 AKI street legal race gas can cost nearly $10/gal and no one has anything better than 91 AKI except for that race gas, E85 is the only economically viable option for raising octane.
2009 335i 6MT
(E92/N54), Black Sapphire / Shadowline (Individual 760) / Black Dakota Leather, ZSP, ZCW, 6FL, PDC, no Nav/iDrive, 19 x 8.5 / 19 x 9.5 VMR VB3 in Hyper Black, Pilot Super Sport 235/35 fr 265/30 rr, M3 control arms + tension links (fr)