When I first joined the Fusion forum in 2009 before getting my 2010, I don't remember seeing a lot of complaints about the MPG, but then again it was June/July and while hot weather will harm MPG it doesn't seem to slam it as much as the cold weather. By the time winter came around there were a few that joined to ask what was going on with their MPG or maybe had joined and seen recent threads about "hey my MPG seems to be dropping"... so I think it's normal, still wonder if the new type of HV battery is more moody with the cold, guess we will see in the Spring if people then mention how much their MPG has improved?
I don't think it's the battery that affects the mpg in the colder weather, it's the fact that the ICE has to run just to heat the cabin, alternatively the AC compressor is electric, so the ICE doesn't have to run just to run the AC. On top of that any car will get worse MPG when it is cold, winter fuel, and just the fact that metal contracts when its cold will effect the mpg, with a hybrid it's just amplified because the ice motor isn't supposed to run all the time. People with regular cars don't notice because they aren't watching their MPG like a hawk. The only mistake Ford made was releasing this car in the winter.