I recently had a P0420 (cat efficiency low, bank 1) on my 00 RS. I have a Stromung 2.25" cat-back, Borla headers, and 47K miles. The mechanic i went to (Honda and Subaru specialist) says he sees these types of codes alot in cars with high-flow exhausts and headers. A lack of heat shielding on the headers can supposedly also cause this, since the exhaust loses too much heat before getting to the cat.
I was told this will cause no harm, but the cats may not be working as well as they could.