I had a 1990 Suburban, and tried some of that stuff. CAI, headers, free-flow exhaust, K&N filter, fuel line magnet (
)... It made it louder. It might have made it faster, but it was still a suburban (= slow). It didn't help with mpg at all. This was before I did any ecomodding or hypermiling, but I have always checked mpg at the pump.
The throttle body spacer *might* help, by lowering the resonant frequency of the intake. It might help if it improves low-rpm torque and power, since that's where you actually drive.