Adding sandwich layer to deck

I recently contracted to have a yard finish the work on my boat that I had started.  After removing the teak decks, Lars determined that the deck flex was within tolerance...but I would like a stiffer deck and do not plan to add the teak back.

Question:  the gentleman working on the deck suggested a layer of 3/16 inch marine plywood glued to the deck followed by 2 layers of grp.  I would have thought some form of foam like airex would be better.  Does someone have an informed opinion?

I trust the person working on the boat but I would like to keep up with the Swan way as much as possible.

Chris Mabel's Casse Tete 43/003