I've got one of these on my bodies and it's fine.. lets buckets of air in with minimal ristriction:
http://www.pipercross.net/competitio...ucts_px500.asp

One of my friends runs the same set-up but got kicked off a track day because the car was too loud.

I was considering one of these though to quieting things down a tad:
http://www.pipercross.net/competitio...600_airbox.asp

There is the pricey option too