Wow dredging up an old one... but I will contribute... I gained 1-2 mpg highway going from 4.0 to 4.7, changing nothing else(same trans, gears, tires). I lost city mpg, although I think most of that is due to a change in driving style. It's hard to keep your foot out of the pedal with a stroker.
The cam you choose has an effect on your fuel mileage as well. A change in cam can mean a change in your power curve, which results in different fuel comsumption for city or highway. For instance, if you go with a "hot rod" cam, that moves your power band higher into the rpms, chances are you are going to drive in those higher rpms more frequently around town and thus use more fuel.
If you choose a low end heavy cam, where your power and torque are in the lower rpms, you may find yourself struggling at highway speeds. It's all a give and take, but overall, if you are trying to justify a stroker for mpg reasons, you are in it for all the wrong reasons.