Engine air-filters:

[Rattnalle]

I will say that cooler air can make a difference in helping your car run better (and less wear)...that's one reason many attempt to get air into the intake that has minimally been heated by the engine bay.

Such as through the original intake located above the radiator

[EcoVehicle]

Are there custom cold-air intake ducts for the C-30 Turbo models?

[MadeInJapan]

Yes!! Here is one company- good but expensive http://www.vivaperformance.com/air-filt ... d-hoses-1/
There are other places to look for less $. Even ebay- maybe Amazon. Haven't checked IPDUSA.com but I bet they have stuff too.

[EcoVehicle]

Here in California, the California Air Resources Board (CARB) restricts cold-air intake equipment.

Any recommendations for specific cold-air intake equipment that receives CARB exemption?

The states that have adopted the California standards are: Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico (2011 model year), New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, as well as the District of Columbia.