My 2014 2.5L GT has been drinking about 0.8 extra L/100km in colder weather. Interestingly, it's actually a bit more efficient in -10 to -15 degrees C than it was at around 5 degrees C. I'm happy, as my '10 FWD CX-7 with the base engine used to drink almost 2 extra L/100km in the cold. Both CUVs are on Blizzak WS-70 tires in the winter (and the same wheels are used on the winter tires, 2004 OEM Mazda3 Sport GT 17" wheels) , so that part of the comparison is fair.
I never warm up the engine or cabin on idle; waste of gas IMHO. I just start the engine, wait 5 seconds and drive. The engine warms up much quicker when the car is in movement, anyhow.