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Well, it kind of depends what KIND OF CAR you have, haha.
Your theory is right on, reducing pumping losses should always lead to better FE. The problem is that if you start fooling the vehicle airflow or O2 sensors, you may end up over fueling for no reason (computer is fooled). Also, slapping a big bore intake and exhaust system could actually reduce volumetric efficiency and thus raise pumping losses at certain / lower rpms. Because engine friction rises with rpm, and fuel conversion efficiency is better at lower rpms, this can mean a loss of mpg in everyday driving. So if you have a N/A car I would avoid a completely free-flowing exhaust. If it didn't hurt your FE, it still sure as heck wouldn't pay for itself in fuel savings.
Now, if your car is turbo / super charged, I would say go for it. They nearly always respond well to bigger pipes.
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