Plastics have been around in induction systems for many years. As Inners mentioned Carter was using phenolic plastic in their Thermoquad carbies from the early 70s to keep the temperature of the fuel in the float chamber cooler. I remember reading somewhere about the mid 90's Ford having a splitting issue with their 4.6 V8s in the US - they used to drop their coolant without warning but that was an earlier design that plastics technology has now caught up with. Had a Mk 4 Golf with a plastic variable runner inlet manifold on it, one of the ways they get better economy/torque by varying the airspeed in the inlet manifold. I'm actually surprised (now that you mention them...) that they seem to have taken so long to find their way into aftermarket!