. . . or can you beef up the hull sides with layers of glass?
Adding layers of glass to the sides offers benefits, but it doesn't do much to add stiffness. One or two layers will increase puncture resistance, reenforce compromised glass and provide an easier to finish surface. It would take many additional layers to really increase stiffness, and all those layers add weight. imagine glueing sheets of paper together. Two sheets glued together (a 100% increase in thickness and weight) adds little stiffness. The paper needs to approach the thickness of cardboard before it becomes significantly stiffer.
Adding a core or stiffeners does much more without adding weight. The stiffeners on my CCP may be a bit of overkill, but they added significant stiffness with less weight.