From personal experience 2 weeks ago on a 6000 mile road trip, I got the worst mileage of the entire trip when I put in 89 for a tank in my 320slow. I averaged ~28mpg when my 89 octane outlier of 23.2mpg was removed. All the other fillups were of 91 or above. The car also had a very noticeable power loss when attempting to climb mountain passes. (think floored in 3rd gear and decelerating at 60mph) I ran that tank down to almost nothing and the next tank was very noticeably different from a power standpoint - no more issues with mountains for the rest of the trip.
The 89 tank was driven with a slight (<6mph) side wind, so wind was not really a factor. I saw 30-40mph side winds through Nevada at possibly-higher-than-legal speeds and still got 25mpg on that tank.
At the cost of two tacos per tank over regular, I'm sticking with premium.