Take the seat belts and other hardware along with the seat and i bet it is more like 50-75lbs total
On my metro the seat belt assembly was 25lbs per belt. That and the back seat together was a noticeable increase in acceleration. I just covered the metal with carpet and it was quiet enough for me.
Weight reduction helps your mileage any time you accelerate so the more often you accelerate in your normal driving the bigger the benefit. Also the car handles better so you can take a curve faster and the brakes have an easier time slowing the car down.
__________________