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...headers typically tend to make the engine run slightly "leaner" because they more thoroughly remove the spent gases (sorta like interia-cleaning EGR) from the cylinders--usually! But NOT always across the engines' RPM band, because at 'some' speeds, the headers won't be "tuned" and reversion can occur.
...if the catalytic-converter & O2-sensor(s) are still there, behind the headers, then the ECU will compensate...making more HP at some RPMs and then making better MPG at others.
...if you consistently drive where MPG improves, you'll get better MPG; but if you drive at RPMs where HP improves, your MPG will suffer.
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