Johan
Ex got M3 in the divorce, waiting for EU Model Y!
Oyvind,
I think you're right. The battery may be thermally insulated from the base of the "pan", but I think an additional thermal blanket would help to keep the pack warm at speed on the highway when the wind-chill could really affect the temperature of the pack.
I have a feeling someone will develop something that can be bolted onto the bottom of the pan, and as someone mentioned could be swapped on and off with winter tires.
My only concern for this is if it would disturb the aerodynamics underneath the car? From my understanding the fact that the bottom of the car is so flat, without protruding elements like exhaust etc, is an important reason for the car achieving such a low Cd(A)? You're already loosing range in the winter due to many factors such as denser air, more friction on the road, winter tires, increased load on heating - so you don't want to loose even more on aerodynamics of the car's body. Also, if there was an alteration of the air flow it would have the most impact when going fast such as highway speeds, and when you think about it, the one time you need the full range of the car is when you're going long distances which you are usually doing at higher speeds...