I've had no losses through the cold!
It's easy to say that when we have over ten hives - one loss over winter is an acceptable 10% - and so no sweat to make that up early in the spring. Different altogether if you only have one or two colonies.
I too have had no recent losses due to the cold, unless 'isolation starvation' can be attributed to a 'too cold' hive. (Not even sure that has happened recently either)
The main use of insulation is to prevent damp by condensation on the underside of the crownboard. Any extra is simply emulating a polyhive, which, by all accounts, is not a bad idea at all. Ignoring polyhives, then think 150-300mm of wood cover in a tree, rather than the nest being 20mm inside the tree bark. I know which I would prefer if I were a thinking bee.
Regards, RAB