The relay control box will not be a meaningful source of voltage loss. You should be more concerned with the wire you use to connect things. For example, 8 feet of 16 gauge copper wire will lose about 1/3 volt at 5 amps:
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I always use a voltage drop calculator when I'm planning wiring, here's the one pictured above:
https://www.rapidtables.com/calc/wire/voltage-drop-calculator.html. There will always be some voltage drop in a wire, but with proper gauge wiring of shortest possible length, you can minimize voltage drop.
As far as mounting goes, the 6-gang unit is smaller and can fit in more places than the 8-gang. I am not currently using the 6-gang unit (or the other 8-gang they sent me to test) - after I testing I put the kits back in the boxes because I didn't have an immediate use for them, but just now I took the 6-gang panel out of the box and posed some photos in one of my 06's for you.
In the center stack, it's a bit too wide to fit on the right side (left photo); it would fit over the left side, but that would obscure the power outlet (center photo); but it would fit vertically on the right side (right photo).
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Around the instrument panel, it would fit nicely on the dash at the upper left side of the console (left photo); also nicely under the instrument panel at the lower right (center photo); or could be mounted at the upper right corner although there it would overhang the panel a bit (right photo).
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It fits very nicely in front of the emergency brake lever down on the center console...
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It also would mount very nicely above the windshield. I'm holding it a bit off-center here because my Garmin GPS mounts in the center and that mount is in the way for the photo.
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Happy to answer any other questions you have about this unit or photograph it (or the 8-gang panel) in any other locations you might want to try mounting it.