Body kits are inspired by the aerodynamic refinements in sedan racing. By careful sculpting of the body panels, and the addition of air dams, skirts, rear spoilers, ridges and diffusers, racers not only make their cars more aerodynamic. They also make them look very different. There are body kits that are very carefully designed to control the airflow around the car, and there are body kits made to give the car a racing look, a look that not necessarily is more aerodynamic than the original but gives a performance character to the vehicle. So there are body kits for show and there are body kits for go.
If your body kit is for show, the aesthetic aspects of kit are of course more important than the actual performance. For a road vehicle a body kit doesn't really affect the performance. Aerodynamics are extremely important at racing speeds, but up to and a little bit above legal speeds, the aerodynamics of a vehicle play a fairly insignificant part in the performance, compared to other forces on the car. Body kits for looks try to imitate the style of real racing body kits, with panels that go down and almost touch the road surface. This is fine on race cars that don't see use on the pothole roads that the state budget restraints are making ever more common all over the country. On a smooth racetrack surface the cars are designed to hug the asphalt and to create a low pressure area under the car, sucking it to the track. On road with a normal amount of potholes, the panels of a body kit can be destroyed quickly if the car is driven at normal speeds and nothing has been done to the suspension.
For maximum good looks you probably want to combine a road-hugging body kit with an airbag suspension, so you can adjust the ride height and ground clearance according to the road conditions when driving and lay the car down on the pavement for maximum visual effect when you park it. And drive with care. Chaffed and broken plastic is not adding anything positive to the looks of a vehicle. Choosing body kits for go is trickier. Especially if you really want to get aerodynamics that win races. That kind of aerodynamics cannot be bought over the counter. Winning aerodynamics have to be constantly tweaked to the specific vehicle, to the specific track, to the changes you make in suspension setups for different races and tracks. In body kits made for show, imitation carbon fiber is okay. In kits made for go, the carbon fiber has to be real, as low weight is a performance parameter that has to be observed just as closely as the angle of the spoiler. So as far as body kits for show are concerned, you are free to choose anything that you think looks good. It won't make much, if any, difference in the performance of your vehicle. A good body kit can however make your vehicle look like it is really fast, which really is the only important function of a body kit on a car that is not used for racing.
In racing, the aerodynamics of a vehicle are one of the key factors for success. It is not about streamlining, as many people think. Streamlining - the art of making a car slip through the air with minimum resistance - is only a small part of the much more complicated field of aerodynamics. Sure, it is important to minimize the frontal area of the vehicle, and to shape the body panels to a drop-like form. But that is just where you start. A perfectly streamlined car would suffer from a lot of problems. It would be very unstable at high speeds, with the streamlined shape creating lift forces that make the car wheels lose the grip. The air would slip by the car without cooling the engine or the brakes. It would be very impractical, with a long and unwieldy tail. Placing the driver and the engine in the right spots to achieve a correct weight distribution and good visibility for the driver would be difficult to say the least. The attempts to make a perfectly streamlined car that are dotting the automotive history make up a very bizarre collection of automotive contraptions. Good aerodynamics have to be found in other ways. Modern sedan racers, which are as close as racers can get to the real world aerodynamics of practical vehicles, feature a number of purpose designed features that break the streamlining, directing the air rather than slipping through it.
The front air dam forces the air to the sides and into ducts that lead to the brakes and the cooling systems for water and oil. Some air is forced through the engine compartment in a controlled way with ducts extracting the air into selected zones where the extra air is needed to create the desired pressure, contributing to front downforce at the same time as it ventilates heat away. Skirts along the sides keep the airstream along the side of the car from disturbing the airstream under the car. Flanges along the hood and the roof separate the airstream over the car and the airstreams along the sides. A spoiler set at a carefully chosen angle creates a downforce on the rear that is just right for the specific track. Belly pans and the rear diffuser control the airstream under the car, creating a low pressure are that helps to keep the car down to the road. The diffuser is also designed to mix the airstream from below into the airstream from above into the vortex behind the car in a controlled way. This is an important contribution to the stability of the car and to minimizing the drag. And all these parts have to be shaped to work together in a way that optimizes the performance of the vehicle.
Fortunately, shapes that are created to make a machine perform in harmony with natural forces tend to be very harmonious and beautiful. Therefore, the choice between a body kit for show and a body kit for go does not have to be a choice between the beauty and the beast. It is more a choice between a kit that provides the best look per dollar and a kit that provides the best performance with cost being of little importance.