I have the Casper's headlight harness upgrade. Installed it along with a set of Hella H4 low-beam headlights.
I thought about making my own upgrade harness but decided to do the Casper's unit because they had a special going on it and I couldn't see saving a few bucks just for the hassle to make my own. Anyway, I'm real impressed with the unit. Very high quality components, a true plug-n-play install. I don't know the specs on the wire gauge or relay rating. But it comes with a plug that you can install that completes a circuit so all four lights come on when high beams are selected. Without the plug, the lights operate in stock mode. I haven't tried the plug yet.
Overall this was a great upgrade. The H4 lights are so much brighter and crisper than standard lights that it's like driving a different car at night. And the harness makes the whole rig even brighter and more reliable.