Yeah....the main reason that Dyneema rope is used in the offroad industry is because it doesn't load up. You can pull it to breaking strength and it will just fall to the ground rather then snap back at you. Its also much lighter weight. I definitely wouldn't say its going to pack down any smaller then a standard rope. That said, you could go with thinner rope and still have the strength you need since this stuff is very strong. The 3/8" stuff does 17k pounds, but even the 3/16" is rated at just under 5k pounds. The tradeoff for me is that if you are using it for a pulley system or a z-drag, its harder to grip the smaller rope.
Amsteel started out as mooring line for the shipping industry, so its primarily designed for water use and pulling heavy loads in an industrial setting. I'm sure it will work well for rafting use too. The other cool thing is that you can splice it, so if you did manage to break it you can fairly easily reattach the ends with some basic tools.
Downsides are its hard to tie knots and have them stay since the rope is so slippery and knots tend to untie themselves. You can still use knots, you just have to take special care to watch that they aren't coming undone. Its great for pulling, but you can still wear through it on a sharp rock. Offroad people tend to have a length of webbing on it to protect it in this situation. Its not good in situations where it gets hot, but that isn't a real concern with rafting. It can break down with extended UV exposure as well. Other then the knot thing, the problems are largely manageable.
Amsteel sells direct and have reasonable prices ($2.48 a foot for 3/8") ... Amsteel Blue 3/8" Synthetic Rope by the Foot - 17,600 lbs - AmsteelBlue