If you look at the pictures of the Forged race car,
Forged Performance: Project 370Z Race Car Build!! , you'll see that there is a lot of space under the panels between the rear of the seats and the rear shock towers. With some creativity and a lot of fiberglass, you could put a nice sub and the amps there. That panel would have to be recreated in fiberglass (unless your willing to cut it up), but it would be almost completely hidden, and you wouldn't loose any usable space in the car.
Time to start shopping for new amps.