[aerohead]

The convention is to block the upper grille opening (as you see with all three Tesla cars),leaving open,only the lower opening,which is at the forward stagnation point and highest dynamic pressure.
That said,there may be peculiarities specific to the Impreza's inlet ducting (or lack thereof) which would require that you keep an eye on your temp gauge until you're confident that you're not inviting an overheat situation.
Cardboard and duct tape would tell you something in little time.