My 1998 Subaru gets +2 MPG (24 v. 22) with straight gas. That’s about a 10% improvement. When I can buy real gas for a 10% or less additional cost, I do it.
It runs better on real gas too.
Ethanol sucks.
I will concur on the 10% increase in mileage.
The owner's manual specifically states that any alcohol in gas should be avoided, and under no circumstances should gas with more than 10% ethanol be used.