Since you haven't posted your diffuser yet, I'll throw this out there:
It is a divergent duct. If the top were similarly curved it would mimic half of an exponential horn speaker. Those couple to the ambient air very well. I wonder whether your speed whisker abaft the rear tire does much. I think pulling the underbody air out to the sides would eliminate a lot of wheelwell turbulence.
Do you think the speed whiskers on the front corners help?
And have you worked with materials like Polymetal/Alumilite/OmegaPanel (aluminum skinned themoplastic)? Thanks for the Plascore link. Here's another:
inrekor™ | ultra lightweight sandwich panel technology