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Isn't the minimal requirement 91 octane for the M? I'm assuming you're stuck in California with their crappy 91 octane gas as the best available? When I lived there I used to buy 5-gallon cans of toluene (which is around 114 octane as I recall) and made my own gas blend BUT that was for a turbo car for track events. Unless you're going to the track or a serious street racer (by which I mean one who lines up against someone else and indulges in license/wallet/life risking speeds) I can't imagine bothering to make a 91+ blend for a naturally aspirated M.
In your case I believe your particular observation probably has much to do with winter formulation gasoline blends (probably the RVP variance but I need to think a bit more about the factors) in addition to lower temps (denser air meaning better fuel burn.)
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