The fact that the car has a compression ratio of 12:1 would favor higher octane gas. The smarts in modern cars and the ability to dynamically change valve and ignition timing and other parameters as well permit you to use lower octane gas but 12:1 would still prefer higher octane.
I can imagine a dual performance benefit of slightly more power AND slightly better mpg, but just how much better the mpg is I can't say and short of a true scientific series of tests it would be very difficult to see that difference unless the differences is larger than about 5% percent or about 1.75mpg.
I'd put the difference in price between the highest octane gas versus the lowest octane gas to be about 7%, and the difference between mid and high octane being about 4%. So, if the lowest octane is below 87 than the real difference would be between mid and high so roughly 4%. If the improvement in gas mileage is greater than 4% then it would be cheaper to use high octane versus mid octane.
Those of use living in mountain regions usually see the low octane being 85 or less than 87 so we need to go with either mid grade or premium gas.