yeah, it should definately have less draw and you could probably get away with a smaller power supply. I like to have a bit of headroom though as everything in the car directly related to the PC, I run off the PC's power supply. That, and it's definately not a good thing to run your PSU close to max capacity all the time as it will greatly reduce it's lifetime (at least this is the case for standard desktop PSU's...I'd assume the same holds true for DC-DC versions as well).
I have an Auzentech X-Mystique and it's great. You can still find the X-Mystique, X-Plosion, and X-Meridian on Ebay. If you're using the Analogue out though, I highly suggest upgrading the op-amps as it makes a very big difference. Different Opamps produce different sound as well, some are crisp/sharp, and others produce more melow natural sound.