Bringing this thread back up because I did a little math for my experience. Crunched the numbers from my Fuelly log eliminating the outliers (first fuel up, pure highway trips, etc) to try and get the most accurate picture of my typical commute routine. My sample size is still admittedly small, but when comparing 89 tanks to 92/93 tanks, I found:
92/93 --- 37.47 mpg (15 tanks)
89 --- 37.45 mpg (6 tanks)
Most of the 89 tanks were Feb-March, so on the one hand still on winter blend fuel and cold ambient temps, but on the other hand I wasn’t up against extreme summer heat/AC usage. The basic idea is the mpgs are nearly identical IME so far. Around here most midgrade is only $.10 more than 87, while premium is $.30-.40 more than 87... so the financial end is really tipping towards midgrade. I have been a proponent of using premium but 89 is really, really close in performance and now evidently mpgs as well. Will go back to 89 for a while and see how things pan out in the summer heat.