The question you have to ask is could a fire develop so quickly that it would take out the MoE before people are able to evacuate?.
From what you describe the answer is probably no. You have cup'd and void detection, in an office environment so anything occurring should be picked up very quickly allowing people to make their escape well before the fire endangers them.
Saying that it does also depends how tall the building is (which obviously has a bearing on how long the building will take to evacuate)
So any measures over above what you have in place I would suggest is only going to be for building protection (unless you have a very tall office building of course)
Building protection wise you can get fire resisting flexible duct but its pricey, you could go for rigid metal ducting with final connection made via flex. Fire damper going through compartment walls is something else to consider.
