a) The Group element's label property is required. I think the option of using a Group just to group elements together (without giving them a title) would be useful.

b) The Section's collapsible property is great for hiding elements from an editor page till the user requests otherwise. Though I find sections too dominant in the editor when used under other sections.
I think implementing the same collapsible property for the Group element (Sapphire's infrastructure permitting of course) would allow a more streamlined user interface.