A belt can be too tight, but normally the belt will just break before it does any harm. Normally...
I believe the 2 main sensors that will really mess up MPG without you really knowing are o2 sensor and engine temperature sensor. Both make the car run rich if not working properly. TPS can as well, but it gives off many signs. You said your dizzy and timing are good. I personally would go with valve adjustment and replace those sensors. They don't always throw codes for some reason. The reason I say valve adjustment, even though you don't hear the valves is because you don't know when it was last done(probably when the motor was assembled by dealer). The least you could do is check the lash. That is very easy to do yourself.