[UFO]

I haven't heard that intake or exhaust modifications as you describe will improve mileage. I suggest taking care of the easy things first, like a good thermostat and a clean air filter. You could find out if you can get a chip tune oriented towards economy, but the TDI engine is hard to improve.

You could focus more on the car itself, and work on blocking the grille, using a coolant heater, possibly adding a lower air dam....that's where I am currently focused.