I don't remember zetecs having secondaries....

Um, I would look at redoing the heads for more airflow up high and low but that can get pricey. Maybe just an intake and a chip would do wonders and would cost a lot less than a new head and stuff.

Hope that helps!
Oh, but if you do intake and a chip you should really do an exhaust system. Doesn't have to be anything special, just one low back pressure muffler should do fine: magnaflow, vibrant, even Walker.