I decided to test this today, as the temperature was -2C in London. First I did the standard 10 min AC warmup of the car - this cost me 1% range (=2 miles on my Trophy).
I then did a 10 min battery warmup - this also cost me 1% range (2mi). After that I did an 18 mile journey, mixed conditions, and the car showed the same miles/kwh as it was doing a week ago,when it was over 5C here (3.1-3.3 mi/kwh), so no drop in efficiency.
What we need to know is the drop in efficiency when the battery is cold. You must know what m/kw you were getting last week, how much less is it this week, when the temperatures are much lower? If you are experiencing a drop of say 3.2 to 2.6 m/kw, that would be a drop of 21%, costing you 4 miles range on a 20 mile journey. In that case, battery warming for 10 minutes should be worth it. If the drop is less, it might not be.