Has anyone got any experience with using cork as a core?  Several kayak manufacturers have started using it recently and was wondering if anyone has any experience with it as a material for both vac bag and infusion?

Yes, I love it. Best way to describe it is as a direct replacement for Soric, but it's better in every way. 1/5th of the resin consumption, stiffer, lighter, higher impact resistance, higher shear strength, lower delimitation, lower print through etc. Can still be used as an internal flow media, although it flows slower than Soric. I see no reason to go back to using Soric.