It's different with almost every service department! Have him go to the Ford SVT dealer where he plans on having his car serviced when needed. Talk to the service advisor and ask them what they think about a CAI or a cat-back exhaust. If they don't have a problem with that, ask about a catted x-pipe. I know some dealers who let smaller pullies slide by under warranty! I've also read of service departments that try to void a warranty for just a CAI.
U.M.
My local service department lets me get by with the mods listed in my sig.