I'm well aware that I have several junctions of dissimilar metals, but I don't plan to let anything corrode significantly. That's why all those joints got coated in anti-oxidant, and why I'll be checking and cleaning them on a regular basis.
And while mild steel may have roughly one tenth the conductivity of copper, there's still a lot of it between the battery and the various sensors and computers you're talking about. It really just comes down to the junction points, and they're sufficient in size for the amperages involved (think about it this way, 1/10th the surface area of that lug screwed into the trunk is still more than enough width of copper for the current involved).
As for the Stillen grounding kit, its primary function is to help ground the engine block to the body. I have it installed in my car. One end does go to the negative battery terminal as well (or did, before I pulled the battery), but clearly its primary purpose is to ensure a good engine-to-body ground, so that a potential difference doesn't develop between the two.
I'm not saying you don't have a point about the cleanliness and stability of the ground reference in the car, but I think you're being overly paranoid about it. If you'd like, I could go measure the resistance of the car body from the current battery ground point to the old one up front for you, and we can compare that to the known resistance of my 15 ft run of 1/0 copper for the positive side, if that provides any insight. I don't think there will be a difference worth caring about.
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