i don't doubt that it makes a difference, i'm more or less questioning the significance of the difference... it's not just the mass, but the location of the mass and it's effect on the centre of gravity.
a fat driver, for example, would have a larger impact on handling than sound insulation and carpets (which are relatively low to the ground).

regardless, i don't really feel that a few kg's is really worth worrying about in cars that weigh 1000kg plus; unless you are seriously involved in motorsport, a difference of say, 30kg isn't going to get you much more than bragging rights.