I agree that if trying to save on fuel it's a a bad investment.
Modern naturally-aspirated fuel-injected engines don't work like old carbureted ones. Reducing pressure drop across air filter can/may increase maximum power, but won't affect fuel economy unless you're always driving at full throttle. Even then the difference in fuel economy is minimal.
For a given amount of needed cruising power, if the air filter has less restriction, then the throttle valve will close that much more to offset gain, so no difference.
I know a lot of people make claims, but I'd be very skeptical.
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