Specifically, a Promoted GM will not have access to your marketplace purchases unless you add them to the game (art and addons) or create the game with them (module). Only you as the creator can control Compendium sharing (if any), and only your compendiums (as the game creator) will be available to share. They will also not be able to access the game settings page or benefit directly from your storage quota increases. They should be able to use most anything else granted by your subscription level.