I don't have any experience with that stuff, but I don't see why it wouldn't work as a replacement. Certainly for the mass backing. The mass backing basically makes your carpet feel a little nicer and reduce noise and vibration, and possibly block some heat. The Hush mat should do the same, only better (not sure about making it feel nicer though?).
As far as the vapor barriers - those are good to have. If you are removing them, make sure the Hushmat doesn't get damaged or anything when in contact with water. The vapor barriers in your doors, as a for example, are meant to keep any water inside your door (from windows, etc.) coming into contact with the backside of your door panels - which most likely don't react well to being wet. Again, as long as the Hushmat can work as a vapor barrier - keep water and moisture from seeping through - then I don't see it being a problem. Just make sure you seal stuff like the OEM had.