Yes I'd agree with that, note even bumping things up to 20F seems to help quite a bit, 30F and higher is even better, zero F also seems to be a break point, colder that that and frequent charges are best.
If you can only make it past those cold days the Leaf is really great, of course if you need to drive 50+ miles in 0F degree days on a regular occasion, the Leaf may indeed be a bad choice.
One nice thing about plugging it in every night in the cold is you can use the climate control timer to have the car be nice and toasty warm when you leave, in that case I also like to use the charging timer so the batteries aren't sitting at full charge for extended periods of time and if I don't need the range I like to use the 80% charge option(not available on your '15 but you can simulate it using the charging timer).
Not so sure if avoiding a high SOC for extended periods of time is as important for battery longevity in very cold weather but I still do.
In respond the update, above asked, I said that I did turn air heat off and leave the seat heat on. On highway with no traffic, I drove about 65 mile/hour, and I never be able to keep that speed. With 43miles total drive, it spends me about two hours, so you can imaging how bad the traffic was.
My distances were also averaging 65-70 MPH(freeway speeds around here) as others have said speed has a big impact on distance, in town or stop and go traffic actually helps things quite a bit.
I am in Toronto, and we use Km, and C. I think most of people here are from US, so I start to use your numbers now..
I'd guess more people here than other sites might understand the metric system(engineers and such) but many are probably like me and know very little
Locals always use C in my yearly vac to Canada and it generally confuses me. I like the newer cars like the Leaf where it's easy to change the spedo to Km/hr to match the signs, although going triple digits(10Km over posted) always looks fast on the dash