I have done this to a few sprinters already.....
The max I have got was 25kgs.... thats doing the firewall as well.
Me mate had try those sound deadening matts that you can buy from the carpet place. And its GOOD !!! a lot lighter.... not that much lighter... but it feels really comfortable in the car.
I am keeping my tar in my sprinter at the moment... and plan to loose 25kg myself.
hahaa, maybe that way I can hear my stero and my car might be quicker.
Also use a oxy, skim through the surface a few seconds... then you can peal the whole sheet off !!
takes only minutes to do the car !!