Josh- Interesting. I don't think I've ever driven my car through any powder. The snow we get out here is usually heavy/slushy or plain bulletproof ICE. The thick sluchy stuff is a breeze to get through with regular tires as long as you don't get too deep and lift all 4 wheels off the ground. I've had the most slipping occur in ICY ski resort parking lots (if you've ever been to Crystal mtn, you know what I mean), even then not really in any dangerous amounts, just when I step on the gas or brakes too hard. As far as hitting untracked freshie snow on logging roads, it doesn't matter what tire you have at all. In deep heavy snow, your tires act like skiis and rely on the sidewall rather than the bottom tread to make you turn. I amazed at how well driving in fresh snow works!! Too much fun! We've tried skiiing behind the Subie with a rope before, but gave up because the car exhaust doesn't make for a very fun time.
The reason I drive on so many logging roads is because we use my Subie as a chair lift. There are many areas here in WA that get snow on a good year, so we go up the mountain and find an open hill with a road leading up it. By far much more fun than paying $40 each for lift tickets and on top of that we hit fresh tracks all day long.