If you tune it properly and ride the same, none.
Efficiency is a good thing. More efficient breathing lets the engine spin easier. Less energy is consumed moving air within the engine itself, less mechanical effort is required to produce a certain amount of power.
Less mechanical effort means less air must be burned, requiring less fuel to burn it. Cruising at a steady speed will take very slightly less fuel than it did to cruise at the same speed previously, with all the restrictions. To realize this gain the carburetor must be tuned perfectly, unfortunately few are.
Most folks will argue this, as they lost mileage when they did the mod. Usually by trying one or two main jets, feeling more power and calling it good, setting the screw at some arbitrary number someone else used, and taking whatever fuel flow change happens from whichever size washer they happened to put on the jet needle. Most riders really don't understand the intricacies of carburetion, some don't even grasp the basic concepts of the different carburetor circuits. And of course there's the wrist factor, nobody does performance mods to a bike to ride exactly the same speed and use the same acceleration as they did before.
If hours and hours of experimentation (days and weeks really for those of us without a dyno) is done to get everything as close to perfect as possible, jets, needles, screw positions, slide cutouts, etc. your mileage could even improve, as the bike is tuned for emissions from the factory, not for maximum gas mileage. They are not the same.
Realistically your mileage will drop just a little bit, as even with all this perfection, if you choose to put forth that effort, in the end there is still the wrist factor.
Last edited by tkent02; 03-11-2010 at 08:17 PM.