Body kits are often described in their marketing literature as being "aerodynamic". And indeed, the variety of spoilers, dams and sideskirts often do look very sleek and aerodynamic. But aerodynamics is a complex subject. It is very difficult to know what the actual air resistance of a car shape versus speed will be without doing actual road measurements or wind tunnel tests. It is also possible to calculate using supercomputers and office buildings full of PhDs.
Very few body kit manufacturers actually have the resources for testing or calculations. The result is that the supposedly aerodynamic kits might not actually lower your car's drag. They do add weight to your car and they do lighten your wallet. So if the goal is to spruce up your car's appearance, go ahead. But if you are making the purchase to save on gas via better aerodynamics you owe it to yourself to double check. Specifically ask the body kit manufacturer about measured drag coefficients. If they can't tell you, then the kit is probably just eye candy like most of the ones on the market.