cwiert, I use and really like a Canadian product (of course, since I'm based in Canada) called B-Quiet (
http://www.b-quiet.com/?fqaflt=rdhgold1&). The price is lower, it's quite easy to apply (though, as flash mentioned, still has the same self-stick frustration of Dynamat) and it's roughly equivalent to the best Dynamat (Ultimate) which is one of the better sheet-based deadening materials.
This is one (as I'm sure you've seen) of dozens of different products available to do essentially the same job: damp vibrating panels. My opinion differs a bit from Flash, though: I find that they're most effective on mid- and high-frequency sounds (like the sound you get when you rap against the metal with your knuckle), while road noise encompases a fair amount of low-frequency sound.
In past installations, I've used a sound barrier material called v-comp (from the same company) with great success. It's VERY heavy, though, so you may not be interested in using it, here. I'm still debating this, as well.
Once again, I'm only providing the theory and experience from other cars, here: I haven't treated a 944, yet.