I don't think the plenum sits directly on the engine. There must be some kind of gasket, specially if the air is coming at high pressure from the turbo.
Depends on how the carbon fibre has been structured in the manufacturing process. For all we know there could be a thin layer of aluminium or other material to act as an air seal (no idea if that would be allowed in terms of the regulations). Also heat dissipation in this case might be a help more than a hindrance given where this part is located in the car.