Best way to step down (half) battery voltage for motors
As @brhans says in his comment, the answer is a buck style regulator. Those can be 90%+ efficient. They are a type of switching power supply that also uses an inductor. A buck/boost power supply can even raise the voltage, but you don't need that in your application.

Buck regulators are quite common in motor circuits, so you should be able to find a 7.2V buck regulator as an inexpensive off-the-shelf part.

In the RC world, such a supply is called an UBEC (which stands for "Universal Battery Eliminator Circuit"). A quick Google search on "7.2V UBEC" found lots of results, including this one:

7.2V "UBEC" regulated power supply