I think the main thing with air boxes is they hold air in them so when air is needed all of a sudden (acceleration) the air is there ready to go, quicker induction. The reason they are carbon is more then likely just for weight saving, adding a big plastic box would defeat the purpose of the quicker acceperation, the weight would probably counteract that added speed

Hope that makes sense.

Japs done one few years ago called "arc air" for Vtecs, 4-AGE engines etc but they went out of business for a while, crazy prices now for them secondhand. Think they are back in business but not making the small and older stuff anymore.

Sorry on phone so ignore spelling etc