As an auto electrician of 14 years i fail to see how adding extra earths to various parts of your car makes any difference what so ever unless your car has significant high resistance and bad connections which is highly unlikely.
Ive fixed many bad earths on various cars, like the earlier R31 Skyline which had a slight voltage on the o2 sensor ground. This caused fuel mapping issues. My point it, unless your car has stray voltage on earth wires caused by bad connections to ground (very unlikely) a grounding kit will do nothing besides lighten your wallet.