As its been below -15 Celsius here for the last six weeks or so now (that's the daytime high, average nightly temp is below -25 Celsius), I can confirm the battery conditioning only kicks in when the vehicle is plugged in, or when the vehicle is running. The only advantage to leaving it plugged in when standing, is that the full regen takes some time to achieve if it hasn't been plugged in overnight. To quantify "some time", it depends on the temperature, but below -20C it takes around 5 to 8 minutes before the regen notification 'bar' starts creeping down the 'available regen' gauge to the right of the speedo, in a similar fashion to when you fully recharge the battery (as opposed to using hilltop reserve, which is all I usually use, less a charge to full capacity around once a month just for shits and giggles and to see what the predicted range at full charge is at this temperature, just in case I need its full range at some time). Worst range prediction (at full charge) that I've seen so far was 214km, after leaving it outside overnight below -28C (-18F) with a min (max?!!) overnight temp over of -32C. That battery conditioning rarely gets above a few percent of the battery usage, as the climate and propulsion draw far eclipses it, other than for the first 10-15 minutes of use, or if you use the pre-conditioning function, but it soon drops down as a percentage on the 'energy use' information graph once the vehicle starts moving.
You can hear the battery conditioning in operation when you plug the vehicle in to charge, if its' been standing for a while in the cold; you can hear the coolant expanding into the overflow bottle as the battery charging kicks in.
The power output is also reduced (limited to 60kW initially) once you're down below -20C. This increases pretty rapidly as the coolant circuits warm up and is not really an issue; I had to push the car harder than I should have done in those conditions to find what it was limited to, as you can't/shouldn't really try to accelerate that hard when it's -28C in deep snow.....lol!