I get really freaking $hitty gas mileage... its tuned up.. etc... what are some things i can do to get better gas mileage.. if anything.. and how much of an improvement would i get swapping to TPI.

 

o2 sensor? Headers....

 

Retard the timing. Stay off the gas pedal. Up the tire pressure. Lower fuel pressure. Lighten the car. Re-gap the plugs.

See a trend here? All of these things (except the last) effect performance adversly. Performance and mileage are two things that work against each other more often than they with each other (that's: "more often", not: "always").