ya, if anything the extra lift (read: drag) would give worse fuel economy. think about it; smoother aerodynamics allow the air to flow around (above, etc) the car. the majority of the work that the engine is doing at speed is fighting the effects of the air/wind that is trying to remain in place (as an example, think of water. a blade running through water will gi faster with less work than a blade with anything, ANTHING attached to it... well, besides a motor ). with that example in mind, anything that is going to be added on to a vehicle to alter lift, etc will incrase drag unless it's used to smooth out transitions.
at least that's with my admittedly limited knowledge on the subject...