An extremely dirty air filter will cause an increase in CO because there will be a lack of O2 to help oxidize the CO into CO2.
You're right jgr7, the EPA doesn't give a shtt.
Performance is somewhat compromised because they are trying to meet emission regulations. They have to juggle to keep NOx, HC and CO down and this leads to a slightly less efficient engine (performance wise). If they were able to have it however they wanted, you would see high NOx and high CO in most of our vehicles. They would also just vent all the gases to the atmosphere, instead of directing it back into the combustion process.