I am not very used to american cars, but from what I've seen and heard, your main problem might be engine sound, not road sound, am I right?
Then your efforts should've been directed to the firewall. Apply Dynaliner, and two layers of the best dynamat there is, and dynamounts to your dashboard and you will probably hear a real difference. Also, this might be of use: get an aftermarket quiet-sounding escape system. That can work wonders on those brutish v-8s americans like so much. Good luck.
P.S. on my Golf IV, the "cover everything" approach worked fine, it's a really quiet car now (it already was, for an economic car)