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?

Perhaps, but you're missing the point. For an infusion application, cork performs and it performs well. My praise was specifically in comparison to other infusion specific bulker mats like Soric and PET core. And of course you can use other non-infusion specific cores, but for me the cork has other benefits that make it worthwhile even if it is a little bit heavier.