I would consider using the settings for the old Watercolor Radiant White sheet paper. It's about as thick as an Epson can take and still print OK. Epson has since changed the name of the paper, but you still should be able to google for the older paper settings, or give Epson a call.
Rule of thumb, for the 300 and 310W papers I use, which are the thickest, I use the wider or widest platen gap. You can run into banding issues with the widest setting, at least I do (Epson 9900), and I only use it when I have to.
Paul