Lead would be heavy yes, but the lead sheeting is very thin and pliable, so you can basically get it to work very well.

I don't think it would add more than 50kg to the car, but for those interested Lead sheeting is a far superior sound barrier than dynamat, dynamat absorbs vibrations by adding mass. With that being said Dynamat is safer and easier to use.

I'm not sure how Lexus get there cars so quiet but I would imagine a lot of sound deadening and possibly lead may be used, that combined with superior aerodynamics.