Plastic grille bar

Aerodynamics make a larger contribution to the efficiency of a Prius, especially at higher speed. The engine runs cooler than a typical automotive engine because it's more efficient.

In the winter time, many Prius owners do a "grille block" where they put foam pipe insulation in the radiator grilles to reduce the amount of air hitting the radiator. This makes the car warm up faster in the winter. The engine uses more fuel during warmup, so reducing the time it takes is the idea. The heat will also work faster. Many diesel truck owners will also block part of their radiators off in the winter for similar reasons.

The aims of most Prius enthusiasts are different from typical car modifications. We mod for economy rather than for more power. If you added a cold air intake- for cooler, denser air - you're also going to have to inject more fuel to make use of it. You'll have more power, but you'll loose mpg. Some people will take a bit of a trade off for more power. Then again you can get a TRD exhaust for a Prius so there's something for everyone lol.