Hmm, tricky. It is not going to be as insulating as enclosed in material as there is an air gap so convention likely to help cool it, but also not really "open" on the non-insulated side. But while that drawing shows an air-gap, it is not clear to me that whoever installs it wont just stuff extra material in because they ahve it, so I would proably err on the side of caution and go with "surrounded" from a thermal point of view.
My other concern is the idea of laying services on the suspended roof - I'm pretty sure that is not acceptable support or likely to meet premature collapse concerns, probably better to have some means of clipping them to the "foil backed plasterboard" or cable tray, etc.
Are any high power circuits going in? Typically if it is just lights then even fully derated 1mm is going to be fine.