I think it really boils down to this: Assume nothing.
Simple math suggests higher ethanol = reduced economy, but the real world tells us that is not always the case. Mostly because not all engines are running at peak efficiency on pure gasoline IMO, but those that are will likely lose economy in terms of strict miles/hours per gallon even if they do burn the ethanol based fuel at least as efficiently.
MY POINT is that you can't just look at the nearly 40% difference in BTUs between gasoline and ethanol and assume that's what the difference will be in fuel economy. There is plenty of data to suggest that will be the case, but it depends on tune!
Based on the roughly 4% drop in BTUs with E10, your economy sucks with a loss of over 6%. It's probably running a hair too lean on E10, causing misfires that may or may not be perceptible, and losing power.
I'll bet your economy comes back that extra 2% if you bump up one octane grade.
(assuming what you say is true)
Why should I pay more for mid-grade E-fuel to get only a 4% drop in mileage when my car runs so nicely on real gas? That would be a double loss in my miles-per-dollar efficiency.
That's the thing. In many areas, increasingly, straight gasoline simply is not available or the stations cannot legally put it into your tank unless it is an RV, boat, etc.
The only stations that supposedly sell it around here are on or near water or other recreational vehicle areas - but I'm suspicious that all of them are actually selling E-free fuel and not just charging extra for the same E10 they sell to everybody else. By our state law, the regulators wouldn't even be interested in checking the content of the fuel since it is not being sold for on-road use.
MSDS sheets tell you nothing... Its probably like "proprietary additive 506" and "proprietary additive 209" and stuff like that... I've tried reading MSDS sheets to figure out what's in stuff before. Never worked.