The only way it would make a difference is on a car with substandard grounding in the first place IE old, dirty, or corroded.

You do realize that all dyno's have a margin of error of at least 1% so anything less than 10hp is pretty much a coin toss as to whether you actually picked up HP.

And finally, I'm sure you also realize that you can't feel HP changes of 10hp or less in seat of the pants testing. So while you guys stick to buying grounding kits, I'll be at home installing my nitrous scented air freshener which should net me similar results.