Most of the studies I've seen have E10 at a 1-3% loss. Some places have E10 all year around.
Here's one but it's from the ethonal folks. I've seen an increase in FE with Eoth in 2 cars, both small displacement. I think in general that the affect of E10 are over estimated and the affects of
temperature is under estimated. YMMV (Your mileage may very)