you would have to take into consideration that different area of the car requires different products (thickness / density) to work best for the area...

a rough idea for you to estimate, a normal sedan(vios/almera/city) cabin full sound proof including the rear trunk area... would take total of 60 pcs of sound proof of various series...

and to your expectations...
it kills the rains noise other than those coming from the windshields..
it kills the road noise coming from the floor pans...
it helps on the audio tremendously...
thats about it to my honest opinion...

but to really reduce the NVH levels when cruising, consider sound proofing and then auto foam your car chassis... that way the wind sound that travels through these tunnels are completely removed and its really quiet during high way speed...

and it comes down to your engine, tyres and suspensions...

so...