On plastic not conducting heat, Um, I don't think it’s relevant.

I have designed a number of air cooling systems,(non automotive, but the laws of thermodynamics don’t really change) and have had to do a lot of reading, research, etc. on materials and heat conduction, versus heat transference in a gas(air) or liquid( water, coolant, etc.) environment.
In an engine compartment, after the truck reaches thermal equilibrium, I think you will find less than one degree C in the air temp from the most conducting to the least conducting material.
This can be verified by a remote temp sensing device. I use laser thermal guns and remote thermocouples. The relative conductivity of the material is not germane. The smoothness of the surface makes more difference to the thermal transference than the material itself.

Not saying the Volant isn't good. You have one, and it sounds like you really like it.
I have the AEM BruteForce and which one is better is based on their flow design, etc. far more than the materials used in the tube.
I am not expressing a preference, just stating the results of a number of years of working with cooling systems, etc.
If plastic was found to keep the air cooler, no vendor would make a metal one. They would make a plastic unit, which is much cheaper to manufacture and thus more profitable.
If they found people would buy them more if they were chrome, well, they chrome more plastic than metal nowadays.
Chris