The "normal" fuel consumption of an unmodified M3W is mid 30's mpg (UK gallons). It often improves with some better breathing. I would check your 19.8 mpg figure as that seems very low. If you are changing something on your engine it is a good idea to reset the adaptive maps with Protune 2, the adaptive maps will adapt to your new set up then over a few miles. A baffelectomy should be within the range of a locked standard ECU.
