If roughly pure ethanol has has 30% less energy than gasoline, going from a blend of 90/10 gasoline/ethanol to a 85/15 blend shouldn't decrease mileage by 25%.
"Due to ethanol's lower energy content, FFVs operating on E85 get roughly 15% to 27% fewer miles per gallon than when operating on regular gasoline, depending on the ethanol content. Regular gasoline typically contains about 10% ethanol."
"Standard" E-10 gasoline contains between 0% and 10% ethanol. E-15 is 15% ethanol. So E-15 has at most 4.5% less energy density than standard gasoline. You would not see a 25% decrease in gas mileage between standard gas and E-15.