As for your provisional choice of pad, the good news is Exped's Downmat 9 ($149; www.exped.com) has an R-value—an insulation rating—of about seven (higher is better). A standard Therm-a-Rest, in comparison, has an R-value of about three (depends on the model). Adding a Therm-a-Rest RidgeRest ($20 in regular size; www.thermarest.com) to the self-inflating pad—an excellent cold-weather practice—adds another 2.6 of R goodness for a total of about 5.6. As you see, that's still some way off the Downmat 9's grand total. So one Downmat more than equals two regular pads—a self-inflating and a closed-cell pairing—in insulation. And while I've never used one of the new-generation down-filled pads, I am told they are molto comfortable.
You also win on weight. A Downmat 9 weighs two pounds, while the two Therm-a-Rest pads will end up at three pounds six ounces. That's quite a difference! You could even add a regular-sized 14-ounce RidgeRest and still have no more weight to lug than before. So while it's true the down-filled pad is fairly expensive—about $60 more than a pair of pads from other makers such as Cascade Designs (inventors of the original Therm-a-Rest)—it performs extremely well. I'd go out and buy a Downmat 9, and look forward to a good night's rest in the mountains.
For a selection of the best sleeping pads and slumber-enhancing accessories, check out Outsides 2004 Buyers Guide.