I can[t speak to coupes, but in the Roadster this is easy, and you can take an inside route. I realized this when I put my 50 watt GMRS radio in the top storage area, behind the seat. While that cable is only a #8, there was a lot of room to run something bigger. I ran it thru the passenger compartment all the way to the fuse block next to the battery. The one questionable part would be the passage from cabin to ECU/BCM/RCM box, my wire was small enough to fit - but a #0 probably won't be. I dont see a BIG deal with going under the car along the chassis - but have never considered it, does it not stand to reason that such a path would be protected and safe in most any circumstance?

I'd be uncomfortable with a fused lead, as your battery is your surge filter and part of the voltage regulation.

I'v done the James1549 grounding mod to my last two Roadsters - therefore, I'd run a #0 or so to the block and a #6 or #4 to the chassis, right in the area around the new battery location.

You sure mod the hell outta your cars, guys talk a big game, but YOU on the other hand.............................