[aerohead]

The airflow direction is a function of static pressure,from area to area.
The lowest pressure on your car is near the A-pillars where the air is accelerating violently around those corners.
If you remove the rear hatch,the air will actually flow from the open rear,forward through the car to the open front windows,bringing exhaust fumes with it.
Since the carbon monoxide won't be improving your driving abilities,it's best to keep the hatch,but do the Lexan if it will reduce mass.