Datapoint for you:
Got a fresh 100% grille block installed yesterday (upper and lower), in addition to the WAI. Drive home today was around 11 miles, all stop and go traffic. Outside temperature ~34F, snowing lightly. Running the heater at a low setting the car was barely shy of fully up to temperature. Air intake hit around 80F toward the end. Fuel economy ~60mpg.
Best image I have, I get home after dark every day:
