Unless it is super hot or super freezing, I would definitely vote for the walk. However, the path is at points not super well lit, and does go under a roadway which makes a bit of a dark part, so if anyone in your group is nervous of walking a slightly dark path at night, then I would avoid it. The walk is about 15-20 mins to the Boardwalk and then about 5 mins or so to the International Gateway, plus a little bit farther to Germany.
Overall, I can't imagine another faster way than walking. To get to the path, you exit DHS and hug the waterway to the left of the bus loading zones heading to the parking lot, and follow the sidewalk which turns into the path. I don't remember much in the way of good signage, but it is pretty obvious once you start. It also gives you a neat perspective of the Tower of Terror lit at night. I would probably leave DHS by 7:30 - 7:45 if your ressie is at 8:30 to allow walking time and any line at the International Gateway, although there rarely is one. If you're early, then wondering World Showcase at the holidays is one of my fav places to be...