Edit: I see you have an 01, I had steady cel issues from intake and exhaust causing catalytic efficiency below threshold. Then I purchased a Milelimnator from pony express and no more light from that code.
I took the car to subaru with 16 miles left on the warranty, and it was the heater circuit in the rear oxygen sensor. They replaced it, and cleared to code. The car actually seems to idle smoother now too, before it was just a little rough.
So it didnt have to do with the CAI or the proECM.