I agree that adding a grounding kit wouldn't hurt if the circuit design is sound. You are dealing with impedance and capacitance possibilities that could degrade and/or delay signal integrity. Most of the time this happens at the connector or terminal especially if exposed to the elements. Cleaning the contact surfaces would solve the issues if you could actually get to all of them. Since you probably can't then a grounding kit could help get your Z back to stock form.
Most electromechanical systems have a specification for point to point impedance levels (usually in milli-ohms) and this is what should be measured since there are too many variables involved with how this may or may not affect vehicle performance. You never know, someone in the assembly line could have used the wrong type of conductive grease and your car never reached it's stock performance level to begin with.
