Yes. Phil (aerohead) has posted aerodynamic studies where OEM's have increased aero efficiency anywhere from 5%-15%, usually resulting in just a couple MPG.
http://ecomodder.com/forum/showthrea...-6-a-2670.html
You would have better luck with an aero topper (see the home page) that has been unofficially shown to improve gas mileage by 20%.
http://ecomodder.com/forum/showthrea...d-cap-583.html
Also look into an air dam made of lawn edging, or maybe an S10 Xtreme bumper/lip. This will push air to the sides, away from the turbulent under body.
But of course you already know that the most useful way to approach this is many small measures adding up, instead of putting all your hopes into one mod.