Out of all the products, I'd use Dynamat Xtreme. Better deadening, lighter than standard Dynamat, and dead easy to use.
As for how much it sound deadens, I didn't have the hand held SPL meter before the work, but my old show car was HUGELY quieter than the standard car I had parked next to it for daily driving.
One little thing, what's with remarks about someone wanting to put some audio in THEIR car??