With more air flow it takes less throttle to acheive points of acceleration. For instance it may take 50% throttle when stock to acheive a certain desired accerlaration, but add the flow mods and that same acceleration may be reached with 35% throttle. If you continue to use the 50% throttle, though, then your mileage will suffer because you will be using more fuel.
Over stock I'd say my gains in fuel mileage are minimal in my GP, but I only have an open cone and catted 3" downpipe. On my Stealth I got 19-20 mpg when stock. When I parked it I was getting 26 mpg on daily driving, 28+ on trips.
