In theory, deadening will reduce flex and vibration, which both absorb energy. That should help with SPL.
For what I have (not an SPL setup), my Explorer metered pretty high (146.9). I credit most of that to the deadening. I covered my entire Explorer, besides the dash, with a little over 200 sq ft. I used 3 layers on all 4 doors and the hatch, 2 layers on the front floorboards and rear wheel wells, and two on most of the roof. I used 1 layer everywhere else, with the only exception being behind the dash. You'd probably need at least another 100 sq ft or so if you really want to do it right.