Hi ALGlover,
There is more about so called “soy based” or other “plant based” foams in post #2 here. They are just versions of polyurethane foams (polyfoam) and I would use the same density guidelines as I would use for any other type of polyfoam.
They are very common in the industry as manufacturers make some small incremental steps to reduce their dependence on raw materials and chemicals that are derived from petroleum.
Phoenix