RE: "air-buggy" - stage one
thats the best prop powered car iv seen so far. as far as more speed, use a lipo batt. thatll give more power and less wieght. for aero, id try making a lid that covered the electronics....one that made the air flow over the chassis. make the cover fit as close to the top of the chassis tub, so it doesnt get in the way of air flowing to the prop.
you could also try mounting the prop/motor so its tilted alittle down, aiming the thrust some toward the ground.