I've got a TS-WX210A in mine, which I mounted to a board that fits in the spare wheel. As bass noises are directional, being under the hatch (and behind the rear seat and below the luggage cover) doesn't seem to have a significant influence on the sound quality. I often have a few bags in the boot and can't say I've noticed any difference.
I've also uprated my door speakers (same size as OE) which helps a bit too
