If you drive it to use all the HP your gas mileage isn't going to be great either. Might as well have fun while not getting good gas mileage than be robbed of HP with a lower octane and not get good gas mileage.
I am here in the snow belt and will be driving my FiST year round. I have 185/60-15 snow tires on it now and couldn't use all the power anyway. I decided to swap my ECU program for the Cobb economy tune (~6psi boost) at the same time I swap my snow tires on. I am also going to run 87 octane til the summer wheels go back on. I expect to average 30mpg+ in combined driving with that setup. Might as well save money in the dead of winter so I can subsidize my 20-25mpg summer
So far I have had the tune in for a couple of weeks. I would guess the car has about 140 HP with that tune. It feels like it has about the same go as my '12 Focus. I don't know for sure but the throttle tip-in seems slower which is good for economy too. I drive mostly county 2-lane roads and I have 120 miles at the 3/4 tank mark. So far I would say it's a great tune for winter or even long trips.
Just filled up. Temperature has averaged 30F here for the last couple of weeks and I suppose we may be on winter blend gas now in my area. Adding in the 1% difference for the snow tire diameter, I averaged 31.6 mpg. That is 80% two-lane county roads and the other 20% split between highway and in town. I have been trying to shift with the shift arrow with occasional runs up higher in the revs when needed. Again this is with the Cobb economy tune and 87 octane gas. If I can continue to average this in combined driving then I will be one happy camper