You might not be able to drive with a blocked grill without overheating, but most people here do not have that problem. When you've got enough cooling for 200hp and you're only using 40 of those 200 ponies, you really don't need a lot of air flow to the radiator...
Upper grilles are mostly cosmetic these days, they usually pull air from lower down.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
often car makers use the upper grille to make the car look bigger rather than add cooling so they are just cosmetic. On my car the grille is open but it only exposes just 10% of the radiator the other 90% is exposed to the bottom opening.
Also, don't forget that mods and driving technique complement eachother; they produce a result larger than the sum of their individual results. For example: The wheel skirts help on their own, and coasting helps on its own, but put them together and you can coast further than before...
I used weather stripping from advance auto parts for mine. make sure to get the type with the closed cell so it doesn't soak up water and retain it. it has more of a plastic feel to it than a sponge feel. i got the generic kind and it was about $15 for a 20 foot roll or something like that.
*edit* in my garage, you can see it on some of my frontal view pics. it is between my hood and bumper and around my headlights. there is a gap in it so that I can actually open my hood. it is also easily stackable. I put more than needed to be there and then closed the hood on it so that the springiness of it also holds it in place.
Be the change you wish to see in the world