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I have a cheap ebay header on my DX, and i gained power all through the rpm range. The best part is the increase in low-end power due to the longer primary tubes. I can now drive at two to three hundred rpms lower without bogging the engine. Lower RPMs should theoretically equate to better MPG. Unfortunately I haven't been able to track my mileage since the install due to speedometer problems. I did have to perform a small amount of porting in the flanges to correct the cheap manufacturing design, but definitely worth $46.
I would stay away from "ram horn" headers, as they're meant to increase peak HP at the expense of low-end torque. Formula One engineers get away with using these because they keep the engines revved up, thus having no need for low-end power.
If you're looking to dump tons of money and time into building a prototype MPG header, look into hydroformed head pipes. Gas flow dynamics are similar to Aerodynamics inside of bent pipes. Round is not always the best shape for gas flowing through a bend. Check out Micron motorcycle exhaust systems. They're engineers had a very good understanding of gas flow dynamics.
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