I've not noticed any great winter summer effect, possibly 1 or 2 mpg in my (current) average of around 45mpg.
I've always suspected the kind of mpg Jocko gets and KJazz was getting is the potential of the car but that most of us, with age, have had some deterioration. Like KJazz I think it's probably a sensor/throttle body/emissions control thing. But I've always found it too daunting to take it all apart to clean up/replace bits.
Of course the other contender for a sudden, significant change in mpg is a sticky brake calliper.
When you say high mileage, how much?
MODIFY I saw your other post, 170k+