Thanks for the ideas. I think I will experiment with the core thickness.
I did some googling around and found a great resource for sandwich board constructions.http://www.fgi.com.au/files/images/stories/pdfs/literature/sandwich_hb.pdf
A quick read of it (see equation 2.6) suggest that the flexural rigidity (ignoring concave and rocker) is close enough to
where E is the modulus of elasticity of the glass/resin material
b= lenght of the section your looking at
t = thickness of the glass/resin layer
d = core thickness +/-
This seems useful as it suggests that flex is proportional to the core thickness squared but on only depends linearly on the amount of glass/resin. My take away is to go easy on the core thickness reduction.