Well from what little I know, the air filter CFM rating would only affect my engine at and above the RPM that asks for that much, usually my peak HP RPM....where I rarely drive the car. Also it's likely that the air filter will flow more total volume than it's rated for as the rating is likely for flow at optimal resistance from the filter material, thus being suboptimal at high RPM's but not actually choking enough airflow that you'd notice it in any but a racing circumstance wherein every pony counts.
In a week or so I'll have my engine dyno'd with my current 500CFM filter and again with a new extreme top filter that flows 900CFM, and will probably see a few HP difference but not enough to make it worth building a custom drop base for the street.
Ask folks who race, they'll be the ones searching for every missing HP - would they tell us they modify the intake filter from a street stock one for optimal performance??
Regards!
Dirk