the place I bought mine had one in a tiny lil 7x7x7 sealed cube for a demo. When we tried it in my car with a 150 watt amp, it's output was surprising.
we tucked the lil bugger back behind the driver's side taillight about 2" away from the rear panel, and I dare say it hit pretty good. It wasn't mind-blowing, nose-itching bass, but for what it was, I was impressed.
that said, I personally won't run anything less than a 10" woofer for a sub (maybe certain 8"s, but damn few). What people don't realize is that a subwoofer's designed to boosy output from around 80hz on down. To get decent output at those low frequencies, you need to move a fair amount of air, and quite frankly, a single 6" subwoofer's not gonna move near enough air to do it with any justice.
if you're looking for good SQ with solid low end but wanna keep your trunk space, I'd suggest looking at the infinite baffle approach. A single 15 or 18 screwed in behind the back seats being fed by a low-powered amp will definitely round out the bottom end without shattering your brain, trunk space or wallet.
the only problem with running LOCs off the stock head unit is, as has been mentioned already, the factory head unit reduces the bass output as volume is increased. Going with an afttermarket amp will help some, but unfortunately, you're only amplifying the existing sigmal, so if the root signal sounds like ass, it'll then sound like ass, only much louder.
just thinkin me thoughts.