Here is a review of the Tesla during winter conditions in Norway: https://www.youtube.com/watch?v=XZ5PqPeOPT0
(Spoiler: Pretty damn good even in harsh conditions)
I had to look up what the 'cold weather spec' package is and seems to be just creature comforts not different engineering/design to increase efficiency in cold weather.
My concern is at -20C in January with a metre of snow, heater on full blast, spinning the wheels as I plow through snow and slush with aggressive studded winter tires how much will that 400km range drop? Even just warming up the interior (a common thing) in the morning and scraping off the ice off the windows before going to work in the morning. My current vehicle's mileage drops in half in the winter due to all those conditions.
You can do morning heating using grid power, trigger it remotely via your phone (they have an app for iphone/droid). I think you also can schedule heating ahead of time.
A trick for manual transmission vehicles in winter when on ice is to shift into third gear to reduce the torque so the wheel don't spin, the Tesla is the insane opposite of that.
[ Come on Elon summon your inner Canadian instincts! ]