Problem you're going to run into put it where the rear seat is the natural spot to put the mounting bolts for the battery interfere with the rear crossmember. The rear cross member runs right across where your butt would be in the seat. I think a better spot to do this is to put it where the spare tire goes or gang two of the smaller Honda sized batteries on their side next to the spare. I was also never a big fan of drilling the taillight and/or rear body panel to put the shut off switch either.It's not so much for the hatch removal as it is for the carpet replacement. I'm waiting for my stepson the weightlifter to come over here so we can get the hatch off. Since it'll take him a day or so to get over here, I thought I'd start work on other things that need doing.
Moving the battery is to get that 45 pound lead-acid lump out of the engine compartment where it's way out front and up high, to inside the wheelbase, and down low on the floor. I've done this before on several cars, and while the improvement in handling is minor, I could always notice it.
I'll make a final decision after I get the carpet out of the car so I can see how to neatly run the cable. Yes, I'm hesitant to 'hack a hole' in the firewall to pass the cable through. I might use a weather tight cable bushing to pass the cable through, but that's TBD for now.