Because people have a lead foot. You don't need those injectors, or fuel pump if you don't get on it. Same with turbocharged applications. If you have a car with a turbo, don't get on it, & shift at low rpm with a stock fuel system it won't mess up, because you won't produce boost so the injectors don't have to put out the extra fuel.
Like I said above, they don't need them if you drive like an old person. & both turbo & superchargers will hurt a gasoline engines fuel economy, usually only one mpg though. Diesel engines are a different story. You must understand they are producing no boost when you are crusing, so they are just a restriction.
Depends on the specs of the parts. A performance cam will definetly hurt your fuel economy.

 

chopper, what is a maf screen, and how do i remove it? and how do i trim/remove the silencer and where is that im assuming near the air filter?