I had an idea while sitting on a couch with a glass of red wine...
Could it be so that the mapping in the efi computer was designed so that in freezing temperatures the fuel mixture is allowed to go leaner, since the cold air lowers the burn temps inside the cylinder and with cooler burn there is not risk of NOx pollutions.
That leaner fuel mixture would explain why my FE seems to be better at freezing temps.
This would mess up your WAI theory a little bit,
Sorry about it.
I have to make further investigations about this...