Can make a huge difference depending on what freq. you're needing to block and where it's coming from.
Download a spectrum analyzer Free App for your phone, run it around the cabin while you or someone else is driving even the road type changes the spectrum.
Once you know what you're dealing with and where it's coming from you can attack it.
I used Mega Mat got a big roll off ebay. took out the seats, carpet, seat belt panels, everything. Laid it along the back wall, two layers on the floor behind the fire wall, 1 layer up inside the doors as far as i could reach, did everything except the behind the headliner.
The cool thing is you could hit the doors with a bat and not ding them.
I'd do it again in a heart beat. In fact i'm thinking of going behind the dash and doing anything i can apply the adhesive backed material too, possibly under the hood just not sure how the heat will affect the glue - may look at a spray version.