As a general rule, if you're going to block part of the grille, leave the center open. The reason for this is that if you have the sides open, you are spoiling clean airflow coming from the very front of the car. It is instead best to pull from the stagnation point and let anything outside of that flow around cleanly.
I'm almost positive there are threads that discuss this. Look up "bottom breathers" like the Corvette. It uses the high pressure zone in front of the air dam to shove air into the radiator.
Or just punch a 25 square cm hole in the lower middle of your grille block, since it won't affect that much.
Or rig up a flap in the lower middle of the block, with a hinge so it can open when you need it. Actuate by cable or solenoid.