Here is a winter trial I ran comparing typical wood Langstroth medium hive vs similar Poly medium hive. Haven't really crunched numbers yet but you can clearly see 
1)  that poly can have a loose cluster in even very negative (-10C) temperatures. 
2) Ti VS Tx @ min point has a ~5 to 6hr lag (plenty of time for bees to adjust cluster) vs wood at less than 1hr. 
3) Daily T swings much less with poly.
No surprises...
I used an artificial cluster made with a 15W bulb setup in the lower chamber.

I pre-heated the top box with the honey next to my wood stove at start of experiment. 

(Still 2 ft of snow and high of -15C and low of -28C today)
2 hives are now brood rearing and my 2 Yukon queens have beautiful late winter clusters. (8 out of 8 so far) Hopefully our last week of really cold...

As always here is my Northern Overwintering Workshop material (always happy getting feedback)
https://drive.google.com/open?id=1_AaeSncad3HqZt1tnGB43lbnOIahzV7z (if attached photo doesn't show you can view it on slide 33)

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