So I splurged on a new air filter a while ago. $34.99 just for a FRAM. Old one didn't look bad but I thought what the hell.
First couple trips out I was hoping to feel some gains, but nothing jumped out at me. But now that the computer has had some time to adjust the air and fuel trims, it's feeling amazing! Throttle response is much much better and certain hills I go up where the truck would shift to get up, no longer need the shift. So the power is there.
Long story short, if its been 20,000 miles or whatever, don't ignore your air filter. It'll probably pay for itself in fuel savings and provide a better driving experience