I did everything JohnC stated above and changed the air box to a "cold air" type, installed a 60mm throtle body (and matched the intake mani), and removed ALL the options, shy of power windows. I also dropped tire size to a 205/60/14, down from 205/70 to optimize gearing; though it added some revs on the highway. My car flew for a na zx.. it surprised a whole lot of people, and would smoke your average ricer on the street!
I did have a pieced together exhaust too, but that might not be legal in cali. There is always the "test pipe" option though.
I hope this helps, good luck.