I can assure you that it is improving your mileage. I keep the "hot air mod" on year round here in Wisconsin. You were smart to do it right away. No sense giving up any mpg. Just remember to remove it when you go in for warranty work. In reality I don't see how it would void the warranty, but no need to raise any eyebrows either. I've forgotten to remove it sometimes when going in for unrelated interior work and have not had any questions asked.
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