This is rather interesting, a few comments from magician.

1) Do you know those in-cabin voltage stabilizer bring absolutely no effect to your battery? Its pretty, can pretend like turbo timer, other than that the major function is to store small capacity to prevent power surge IN cabin accessories. Such as radio, cabin lights, dashboard meter. So, even if u have a huge VS in your engine bay, you still need a small VS to supply your head unit.

2) Do you know the whole car's grounding work are connected by the chassis frame work? Therefore when you step on your brakes, your brake lamp took electric from batter, and the negative back flow will have to go through your entire boot section, pass back to chassis frame/roof, go through your firewall, entire hood then finally channel it back to your battery. Nobody able to justify how much power loss in between!

3) Do you know simply putting ground cable everywhere can improve the grounding work? I supposed you know, but the extra ground cable only send CERTAIN amount back to battery -ve pole. The rest of the discharge still have to go through huge obstacle(chassis frame) before going back to battery. Theoretically, if your car's ground work is already optimal, by adding extra ground cable can mean create more obstacle for the ground flow. Its similar to when u driving in federal haiway with a band of GaGa kaki convoy, some choose Jln Universiti exit(assume this is the chassis grounding), some choose Sec 11(extra grounding) route, some get out from Hilton(another ground cable), then a small group diverted from Sec 14 get out from Sec 17(from radiator tank pull 1 ground cable to engine rocker cover). As a result, those reach earlier in SS2 will have to wait for other smart asses for a wonderful TT.

Therefore, unless chassis grounding insufficient(traffic jam in Jln Universiti), by putting extra grounding might cost negative effect.

Info source: H&K Accessories, Dengkil, Sepang
P/s: The above input is based on magician's personal opinion, and may not represending the whole truth. I just find it interesting and wish to confirm with our electrical/electronic engineers in the forum. Cheers.