Sorry mate, I can show you just as many studies which prove otherwise.Mention this before, But will say again.
Found a big study in the US on 350z when they came which the test was how much improvemant did occur when fitting a voltage kit on it.
The results were an increase in BHP figures of between 5bhp - 20bhp on the 10 ten plus cars they tested. They had different level of mods and were put on a dyno before and after to see the results.
Sorry mate, I can show you just as many studies which prove otherwise.
Like NickM said, were all other factors consistent between the tests: e.g intake / ambient temps? Barometric pressure? Tyre pressure?
At the end of the day, a grounding kit generally would make that much difference if the car's current setup is inadequate. If you have frayed / missed / damaged earths, then yes replacing them would make a significant difference.
snow: that one (usually sold on Ebay) is not a genuine Nismo product. You'd be better off getting several lengths of decent gauged power wire and wiring it yourself. Main one to consider would be the length between battery negative and ground.