E10 is stable for at least a few weeks, and the Insight vigorously captures fuel vapors, so it really shouldn't have anywhere to evaporate to.
But if alcohol DID evaporate out of your gas, you'd be left with a fuel with more kJ per gallon (how's that for mixed units!), so you'd burn less of it.
I'd wager an improvement in your mood not unrelated to getting a 93.5mpg tank improved your perceptions of your car's speed and performance.
