are they rated to a certain levels of power / air flow

im looking to make an air box and poss using 2x rs turbo panel filters

or do k+n, pipercross etc etc make an air filter to any specific sizes?

 

they are rated for bhp/airflow.
i once worked on a well known subaru that had thousands spent 20-30k on it when i removed the front bumper it had a standard k&n cone filter and you could see it had been sucked out of shape,it could not flow enough air.........he knew everything so i stuck the front bumper on and said nothing.