What other benefit is there to spend the kind of money you're about to just to get about 100+ more miles out of a tank? I just don't see the point.

There's a lot more you can do with the trunk of the car than to add a gas tank. Put a custom system in the trunk if you're not going to use it. That's a little more practical.

But hey, it's not my car, you can do as you please with your property and money.