I think plastic foundation is the least of your worries if you are using wooden Langstroth hive with upper ventilation.
Just FYI I only have reduced bottom entrances. I do add a piece of styrofoam insultation between inner and outer covers. That has solved any condensation issues that I had in the past.
What matter most are the winter bees. If the plastic frames do not work then beekeepers many years before us already complaint about this issue. So far they have not!
I understand that a strong hive can overcome many of the obstacles we put in front of it. It is the marginal ones that I was wondering if the extra thickness of the plastic would be a problem. If it is a problem then it could be that the beekeepers just don't know about it. That could explain some of the winter losses if it is true.
You do not understand heat transfer. Two bodies at the same temperature do not share heat. There is no heat transfer. What happens is the area that is exposed to a lower temperature is decreased. An analogy would be your dining room and living room that have an adjoining wall and kept at the same temperature. There is no heat transfer between the two rooms however being joined together they each have one less wall exposed to the outside temperatures. That decreases heat loss.
Wouldn't it take more heat to keep the living and dining room the same temperature if the wall was 5 ft thick? You do have to keep the wall warm. The "wall" in the cluster may not seem thick to a human but to a bee?
Insulation between them works just as well as no insulation when they are both generating the same temperature.
If the cluster is typically in the shape in a ball, then each side of these "walls" that run through it around the edges are not at the same temperature.
I don't know have any answers, just questions.