[wilf]

Mark - the general rule is "if it ain't broke............."

To be doubly sure, get a high quality voltmeter, set it on a range to show 0.5 volts or so, and check what voltage appears between the engine block and a convenient part of the chassis when the alternator is charging hard (e.g. just after startup). You should see about 150millivolts ish.

Anything in the range of about 0.5 Volt drop means the earth lead has too high a resistance, and could do with being replaced with one of larger section, and the connections checking thoroughly for cleanliness.

Even if you had three or more earth straps, I don't think it would improve the performance of your car one iota. In fact the actual extra weight of them might slow you down!