Aftermarket hoods add a huge improvement to the styling of your Mustang. Hoods go past just looking better but they also have various performance advantages. There are three basic types of hoods. First you have hoods that are taller than factory, these are usually to accommodate the addition of a supercharger on top of the motor as the cowl or raised area of these hoods provides more room for additions to the motor, like power adders. Second, are hoods that have some type of scoop that will ram air into the engine compartment. Some will even dump the air directly into the air filter. The actual amount of air that gets forced into the engine compartment will vary by the size and placement of the scoop. The faster your Mustang is traveling, the more air actually goes right over the hood and hits the windshield, thus resulting in less air actually getting rammed into the scoop.
The third type of hood you can get for a 2005-2014 Mustang is one that vents heat from the engine compartment. This type of hood is sometimes combined with ram air scoops as well. Vented hoods most likely give your Mustang the greatest amount of performance gain. Cooler temps result in your engine making more power. Some of these hoods have vents right on the top and some vent it directly out the back of the hood. The hoods that vent from the top usually have some sort of catch pan for water, thus enabling you to use them in all weather conditions.
Another performance gain from aftermarket hoods is that they weigh less than a Mustang's factory hood, depending entirely on the material. Fiberglass is not much lighter, but carbon fiber and plastic will be much lighter and significantly reduce weight. Any weight savings translates into a faster car, especially when removing it from the front end of the car.