Primeval Force gives 5% elemental penetration, and also Forces of Nature has 6% elemental penetration. If I'm already going for Primeval Force, how should I value also taking FoN if its nearby - is it better to stack penetration, or just a little bit goes a long way and adding more will give diminishing returns?

Does the value of getting FoN change if other cluster nodes are also considered (FoN cluster gives less bonus than PF).