Here's one data point for you.
I fully blocked my upper grille with fiberglass and epoxy resin laid up over pink styrofoam insulation. The lower grille block is just fiberglass wetted out and laid up over the bumper, with no mold. My license plate is a movable aerodynamic device.
In this picture, the plate is in its winter position. It's good up to around 85°F on flat land at 60mph, or long gradual hills at 74mph and 65°F.
I moved the license plate to the passenger side of the bumper, exposing another 0.5 ft² of grille, and I haven't had overheating issues yet, not even on a fast interstate trip on a 90°F day.
Do keep an eye on your coolant temperature, though. A radiator fan won't save you when you're going 74mph. In fact, it doesn't really do much at all when you're going that fast.