I had my 15" Eclipse facing up. I was running like 1000 watts rms to it with a cheap Visonik amp. It was plenty loud and like most systems completely over powering. To be honest with you, unless you plan on going into comps I wouldn't stress over the sub placement too much and would do it the way that is the most functional for the amount of trunk space you need. Sure, if you don't need the space at all go nuts but I did, so I built a box way in the front of the hatch facing up, the amp was built into the same cabinent right beside the sub. It sounded loud and pounded like hell, which is what I wanted at the time. Anything you put in your trunk, no matter which way it faces, will impress most people, because they don't know what sounds good and what doesn't.

97 RS- Runs, but never driven
2000 Subaru Outback- Daily Driver