[renault_megane_dci]

I am in automotive business since 2001.
I joined this site 2009.

Seeing current passion for this thread, In only wonder why you use that tone with me.

On a Renault (I work for them in engineering before you ask) request for AC automatically set a timed run of the cooling fan. (Obviously the heater fan is on indeed). That is a fact.
I assumed it was the same on most vehicles, including yours.

As for temperature, I stated my opinion comparing long enough run when it's colder for the car to still be at operating temperature.
I know what is required for power : colder air.
My point is since we have plenty of power in our diesel cars, maybe hotter air acts like downsizing power.
The proper way to make use of more power being through taller diff ratio. Since this is not the most economical mod, then downsizing the power is IN MY OPINION a nice way to achieve the target of using less fuel.
There is a direct link between your engine RPM and speed so the idea is to use the least fuel for a given RPM. The tuning you had done is to increase the amount of air at each rpm but I don't know if the ECU has been tweaked to deliver less fuel proportionaly to the amount of air (there is an air sensor by the way ...).
So I know there are various reason why overall FE drops in winter but I believe a reasonnably hot air pick up should help and not only for warm up.