I have spent some time doing sound deadening under the dash, against the firewall:
Layers of KnuKonceptz Kno Knoise CLD and Second Skin Luxury Liner Pro CCF/MLV combo. Plus some egg-crate style foam stuffed in the cracks and crevices.
It has made a remarkable difference in reducing the engine noise! Next I will deaden the hood...
Started sound deadening the hood a little, this too reduces engine noise in the cab...
I am putting cutouts in the recessed areas and then covering the hole thing with a single piece.
Last, I'll tape the edges with aluminum tape.
Overall, the car is much more quiet on the road than before... With the combination of all sound deadening applications, it's a night and day difference.
It cost me just over $450 for all the materials, and to me, it's a worthwhile upgrade...
I was getting an occasional rattle that I narrowed down to the spoiler, so I pulled it off today and deadened everything that could possibly move or vibrate...