To me lots of things will determine overall strength. And all these factors work together as a system to provide strength.
*Full length stringers properly installed in a boat.
*Quality and type of glass used
*Type of resin and how its utilized to bond the layers (properly catalized and given enough time to cure)
*Seperate open bow mold not just a cutout.
*How well the top and bottom are bonded
*How bulkheads and other supports are engineered/installed
*How are the gas tanks installed (Just glassing them to the hull sides are not a strong as building seperate supports that the gas tank bolts to)
* I think foam may help keep a boat quieter but I don't think it adds much strength. In fact, since it absorbs moisture, it could lead to rot.
*Whether the floor is glassed over (stronger) versus resin flo coated.
etc....