EGR if done correctly
can increase fuel economy by aiding to the vaporization of the fuel before
it gets into the chamber. However, as stated above it adds nothing to the combustion process and usually ends up decreasing MPG and performance by cooling the burn (actually makes less fuel burn, so it burns cooler).
The key to good fuel efficiency is combustion efficiency. High compression with an efficient combustion chamber design, good fuel vaporization, and plenty of turbulence in the chamber (goes hand-and-hand with good cylinder head design) will make the burn quicker, dump less heat into the engine, give you more power and
economy. Less ignition advance will be needed for peak horsepower.
If you're considering doing work to the engine to get better gas mileage, you may want to consider "edging" the chamber, and also installing "Singh Grooves" to the squish pads. These two modifications have been shown to greatly reduce detonation tendencies and therefore allow you to run higher compression on cheap fuel. The Singh Grooves increase chamber turbulence and help fuel efficiency by themselves. The smoothness of the chamber after edging also helps speed the burn, as there are no "speed bumps" to slow it down. Doing just these two even without raising the compression will help your thermal efficiency. They just happen to have the nice side effect of decreasing detonation and therefore allowing you to run higher compression than you normally would be able to. Compression is the key to thermal efficiency.
These two modifications are cheap (free if you do them yourself) and have been proven to work. They'll require a lot less experimenting and fabricating, as well, compared to building an efficient EGR system.