Are you sure about the mileage factor? I mean what was your procedure for figuring this out? The reason I ask is because if it was cooler for one than the other then your research is flawed. Personally I don't see that much difference in my mileage, everything else remaining constant. I don't have a problem with temps changing on me ( although I am in Michigan where temps can and do change quickly at times) because it only takes me two days to empty a tank of fuel. In the Rabbits I get better mileage on biodiesel than on Petro-diesel.
I have noticed in heavier vehicles, My Mercedes, My dually etc. the mileage is either the same or only slightly lower like 1-2 mpg.

The reason I have to mix in cooler weather is because I only use 15% Methanol. This makes the cost lower and I can justify using a little diesel in the mix for a couple months out of the year. Besides, I could always put in my second tank and preheat it, like I do the straight veggie oil.

There are many different ways to go about it, you just have to look at what works best for you! Tom