First of all, I guess you are talking about the special theme
"Heart of Winter" and not about the spell
"Murdering Winter", right? (If so then might consider to edit the subject of your original post, otherwise I shall consider to delete this post...

)
Second, when discussing the temperature scale, one should be aware of the effects of seasons, which may alter the climate scale significantly.
That said, there had been a few discussion about whether "Heart of Winter" actually works or not. Some people claimed that "Heart of Winter" is a necessity, some people claimed that it is bugged and that it shows no effect whatsoever. A few were undecided on that matter...
I cannot recall how the discussion ended, but my personal opinion on this matter is that "Heart of Winter" is an utter waste of design points for a Cold-3 nation having a high dominon strength (i.e. 8-10). I assume that heart of winter might be useful (and maybe only noticeable) for very low dominion scores, but I never took the time to find out for myself...