I'm actually in the process of deadening my car right now, too, and upgrading the audio (Precision Power 3-ways up front, stock rears for fill, Stereo Integrity BMI MkIV 12" sub in the spare tire well). I figured, if I'm planning on keeping this car for the next 15-20 years, which I am, I might as well make it comfortable, and part of that is making it quiet.
This will be the 4th vehicle I've deadened, and one thing I've learned from that and others' experiences is the end result is completely dependent on how OCD you are on the installation. Spectrum is a vibration damper, designed to turn the energy of vibrating metal panels into heat. It's also very light, about 1/4 lb per square foot. It will lower the resonating frequency of the panels you apply it to, but I don't think it will do much to quiet the car by itself. All the marketing blather on Second Skin's website about this single product creating "tomb-like silence" is just that--blather (nothing against Second Skin; I just ordered a pack of Damplifier from them, in fact). There's a reason they and other vendors sell a variety of products; to make any noticeable difference in noise levels in the car you have to go with a layered approach, add as much mass as you can in the space available, and cover as much area as possible leaving as few holes as possible. There's a reason very quiet cars, like the Lexus LS or Mercedes S-class, are heavy and have solid doors.
In my Prius, I've started by putting vibration damping mat on the outside door skins, sealing up the access holes with acrylic panels with Damplifier on them, then a layer of acoustical foam to decouple a layer of mass-loaded vinyl, both of which cover the entire door with holes only for the speaker, lock switch cables, and window wiring harness. From there, I'll move on to the floor and hatch with the same layering approach. The car will end up weighing about as much as a Plug-in Prius when I'm done.