Exterior soffits have to be designed specifically to suit the site conditions, in particular the wind loading on the soffit. The wind creates both an upward pressure and also a downward suction pressure........ usually the grid behind the soffit liner has to be calculated specifically to suit the loads, so if the spec calls for hangers at 300 ctrs then that has been calculated that way (although it does seem a bit extreme). Have the support channels and fixing cahnnels been set a strange centres as well?
Where is the project?
I am the Knauf Aquapanel Exterior Specification Manager and get involved in specifying these sorts of things fairly regularly...... It is not common to have just a 9mm masterboard, a 12mm board is more common - this may explain the structure behind......
If you need any specific assistance, or have any further semi exposed exterior soffits, my contact details can be found on my profile.