Steve,
In terms of weight, its a minimal addition. In terms of sound proofing, it's really difficult to tell anything quantitatively, since I never flew without it. I THINK it must be doing something but don't really have a direct comparison. Flying in other RVs with various configurations seems about the same, and maybe slightly noisier than mine, but I also use ANR headsets. Obviously there are multiple other sound paths than through the firewall. Bottom line is that I'm happy with the installation and would probably do it again, since it was quite easy to make up the layers. The one thing I would modify if doing again would be to put more glue between the layers when building the "sandwich." There are a couple of places that have delaminated slightly and I'm sure its for lack of sufficient glue. Is it better than just a single layer of super soundproofing that is equal thickness? I don't know.
In addition to the firewall, I put some soundproofing on the floors (super soundproofing 3/4-inch thick, no sandwich) and laid some 0.063 Al on top of that material, which also probably helps with the noise attenuation. I think that the benefits (if any) of the sandwich material are not likely to be as important on the floors. I'm considering doing this under the seats and baggage area as well (one of these days.... maybe at annual time). And eventually some interior cloth panels will help.... but I'm too busy flying to get so some of this right away...
Sorry I can't be more definitive on the sound-reducing properties.
cheers,
greg