I think its great that you got a reply from BMW, and not just a stock one, but one where engineers were spoken to.
However, I have long had doubts whether a higher rated thermostat would improve efficiency. I changed mine last year and my original temp was in the 60's (guage on 1/4). A rise of 20c, has not made much difference to mpg, if any. I doubt whether adding another 4c or more will be the answer to more efficiency. But I think its good for people to try.
Matt
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