What's he doing, adding sport seats to his car? Man, the factory one looks real good. If you can get them and they fit it would be fairly easy to have the stock rear seat redone to match the fronts. Our rear seats are very specific to our cars so I doubt the Tiburons would fit in the first place.
Can you put the sub's flush in the rear floor portion? I bet you one sub in the rear will get louder than 2 in the rear seat area. The whole top portion in front of the gas tank could have the amps flushed. You could then have removable panels or even motorized lids to show off the goods. I see a lot of cars with stereo gear built into a false floor. It's very stealth, durable and functional if you need to haul something.