Everything involving the electrical systems in your car requires a free flow of electrons to (+ve) and from (-ve) the battery. Good electrical performance requires a grounding system that is able to take a lot of current with minimal resistance. Your spark plugs, lights, fans, ICE components, air conditioner, power windows, cigarette lighter, etc. all work on a complicated electrical network that includes fuses, relays, and electrical cabling and they all share a common ground.
However, a common stock grounding system consists of just a few non-braided cables that connect the chassis, engine and firewall to the negative terminal of the battery. As these connections oxidize or come loose over time, the amount of conductive material decreases, and the electrical devices have to work much harder to overcome the increased resistance.
Even brand new cars can have problems with inadequate grounding. If you want the ultimate in everything, you should consider getting a set of high quality grounding cables that help make sure that the electrical equipments in your car perform as well as they can.
**in short, grounding WILL NOT improve performance. It helps restore the lost power due to improper grounding of the stock cabling system. On a new car, it is an insurance for the electrical equipments to work properly.
This is from someone selling custom-grouding set, sincere enough?