Spreadsheet showing MPG over the first 7 months - 5192 miles.
General observations: Notice the MINI's OnBoard Computer (obc) almost always indicates 1-2 MPH higher than the spreadsheet calculations. Also notice that the 14 tank average for the first 5192 miles is 35.1 MPG.
Mileage from tanks 6, 7, and 8 (green) were all Top Tier Premium, AKI 91, which delivered an average of 35.18 MPG.
Tanks 9, 10 and 11 (87 AKI Regular - orange) gave me a 3-tank average of 34.46 MPG, so Regular delivered 0.72 fewer MPG than Premium - but during colder weather.
I decided to extend the test to get more winter data from AKI 91. Tanks 12, 13, and 14 gave me a 3 tank average of 35.26 MPG beating the 87 AKI average by 0.80 MPG.
Bottom line (so far): Premium is giving me a little better mileage than Regular, but only by about 2.2%. As of mid-March, 2011, Regular is selling for about $3.23/gal in my home town, and Premium is about $0.20/gal higher. So I'd pay about 6.2% more at the pump for Premium to get 2.2% better mileage. If cost was the only criteria, looks to me like Regular is the way to go.
Of course cost is not really even an issue when the total difference between regular and premium is only about $2.50 per fillup.
My subjective opinion, is that the car is a little more eager running Premium, but not by much. Then again, I haven't been driving the car very hard this winter because my daily commute is not long enough to get the oil warm - a hard driver might notice more performance difference?