Sound Deadening (how much difference)

[SCOTI]

Another option (layer) that high-end vehicles use (OE production type) is Mass Loaded Vinyl.

I used the foil backed butyl as the first layer for temp isolation & noise/vibration cancellation.
The layer of closed cell foam as the 2nd for sound absorption.
MLV as the next layer for sound isolation.
Carpet.

My dually has 1.75x3" full length headers w/3" In/Out Dynomax 17749's under the back seat (crew-cab). The exhaust piping reduces to 2.5" tail-pipes w/45° bends angled down under the rear bed corners. It will set-off car alarms cruising through a parking lot if you get close enough but is as quiet as stock restricted exhaust from inside the cab.