I'd say it makes a big difference. I remember before we tore the car apart the inside of the car sounded like a tin car. Touch the roof, sounds like a tine can....close the door, sounds like a tin can! That's gone now. It also reduces the sound from the engine a lot. Although, we do open our windows to get that great mopar sound.
BTW, I went all the way up the firewall on ours.