No no no
no NO!. Optimas don't gas under normal conditions, but that doesn't mean they don't
ever gas. Since your battery is located under the back seat, there's a real danger of explosive hydrogen gas building up in that area. And to make it more fun, in case you didn't notice, there's a fuse block right next to the battery!
I've had an Optima red top for about 5 years and used it in 3 different applications. It's currently in the Jetta. I love it, but there's no way I'd put it under my back seat.
I hope the battery you bought at Costco is properly vented.