it probably depends on the car's upkeep.
If its driven dry days only, kept spotless from day one, has a showroom condition engine bay, chances are it wont benefit much.
if its an all weather car, wiring is a bit old now and used more than cleaned, then earth leads n contacts will probably have a bit of corrosion n the car may benefit from an earth kit.
I think mine falls in to the latter category so an earth kit is on my list of bits n bobs 2 do some day when I work out how to print money
