I bought the kit from Kascar a couple of weeks ago and installed. Two things: My grounding was on the passenger side head. I went ahead and grounded to the drivers side head. Second thing: The gauge of the wires appeared to be "lighter" than what was shown. I don't think it makes a difference in the long run.
Ground wires generally don't need to be as heavy a gauge wire as the hot lead.
Current only needs to "sense" ground so it "knows" which direction to go, otherwise it sits in storage (battery, capacitor) waiting.
I found it very interesting that the 30 (THIRTY) amp-rated (might be higher, but that was the largest circuit breaker in in the kit) Power Distribution Unit (found on many later models in the battery tray/box under the commander seat) has only a 16 (maybe 14) gauge wire to go to ground.
Makes a lot of sense when you think it through. Current is the flow of electrons from high potential to lower potential (ground) intended to convey the stored energy to something which converts it to something else (light, heat) through resistance. If all that energy is flowing, lighting up the road, there's NOT a whole heck of a lot of energy returning to ground, or shouldn't be. If there's TOO much the fuse or CB breaks.
Ground wires? You're probably fine with 14g, even silicone spark plug wire (insulated for what, 30kV?) is probably overkill.
NB