First you should ditch the sub and wheels. Go back to stockers. Oh and if you've got the stock front bumper, put that back on. Then conduct the 6P's you probably could use new O2 sensors and have at least one boost leak. My TT get's better gas mileage than your NA.
"The 300ZX Turbo is a dance; it's a song; it's rolling, roaring automotive art. There is no color that doesn't suit it. There is no mood-lifting chemical substance it can't replace." - Automobile Magazine