Uncle first off IF you are building a boat for longevity Wood/Plyrot/Balsa is not on the input ,home depot-shopping list! Where the Plastics can be exposed to water and humidity permanently.
Now where the Behavior of the sandwich Structure under overload, Impact, fatigue. Vibrations and ETC the Plastic wins hands down.
Stiffness again Uncle not true by any measure in a well-designed panel! If anything it will be self-destructive. As if wood is used that does not have the consistency of Plastic.
And again your Data is NA where the Liner foams are concerned. Now off topic the best in Sport fishing Boats and Composite Yachts are built using Plastic Cores.