It's really not as bad as you guys are making it sound.
The hole was cut VERY cleanly. The metal plate will be scoured and then primered/painted on the trunk side, and rubbercoated underneath once the cell is strapped in and sealed. The entire idea behind the fuel cell is to reduce weight, give plenty of room under the car for access to the rear diff and exhaust, and clean up the nasty, cancerous looking glob of metal once formerly known as the stock fuel tank.
Functionally, I'm losing my spare tire along with any real or usable trunk space, yes... but the benefits far outweigh the downside. I don't plan on hauling any cargo aside from the widgets and things I'll manage to toss into the back seat - and I've got AAA Plus should I ever blow a tire.