The one being sold on ebay... you can get that same kit with 5 different brands of stickers on it for under $15+ shipping. HKS, Greddy, I've even seen Spoon. I highly doubt they're actually authentic products from those companies because no matter what sticker is on them, the parts are the same. I'm sure there's no different technology between them even if they are authentic. It's just a ground kit. Electricity doesn't care what brand of wires it's passing through.

As for the P0420 code, do a google search and you'll find that either a bad secondary o2 sensor, cracked exhaust manifold, leaky exhaust, or bad catalytic converter can cause this.

You know how to find exhaust leaks, right?