That someone would compare Lead to Sorbathane, i.e. a soft metal to a visco-elastic polymer makes me really question what message they are trying to communicate. Lead would be more an analog to granite, SONEX an analog to a GIK acoustic panel.
To experiment properly with Sorbothane, one has to properly implement it, which means using the correct hardness material as well as the correct cross-sectional area for the weight being supported, no different from a spring. Unfortunately, people just say "Sorbathane" or spring, without the required knowledge to properly implement and hence they lead people astray.
SONEX is an acoustic foam, so not sure why you are bringing that into the equation ..... however, just like Sorbothane or a spring, it must be properly implemented to work well.