If I recall rightly, this has not got too much to do with weight, yeah a couple of hundred litres of lead based paint would add weight to the Mustang but this is more to do with aerodynamics and the orange peel effect in some older paints, which caused irregularities in airflow over the surfaces. For a long time it was a known fact that certain super-car manufacturers painted their cars with the same paint used on fast jets to minimise the chances of the normal paint causing a similar effect on speed and air flow.