running lean, bad mpg and low power

You would think bad fuel mileage would show up on Long term fuel trim figures, I have seen a lot of cars at zero or +/- 1%. Lexus loss power if air flow meter goes bad and fuel consumption increases. I hate replacing parts without proving they are bad, I would want to see Air Flow meter voltage or grams of air flow readings on a scan tool with a known good air flow meter and I would then look at O2 Sensor readings and coolant temperature sensor reading to see ECU is reading good figures.