No clue, I have never spent money to try to gain mileage, only power and performance.
Also, can you be sure that 600 miles is enough to get an accurate sample of mileage? Conditions change and as you said, gas is probably switching to a winter blend. Checked your tire pressure too? So many things can be to blame.
Why don't you call vmp and see what they can tell you? Or return the truck to stock and see what happens.