What I found is, if you try to invoke G96D800S450M3, from MDI, it starts at 800 and stays there, at the cap speed, regardless of where the x axis is at, if and when you go back to MDI, and enter g92x.....z....., then it will automatically kick the spindle down to whatever RPM it should be at for the given diameter is, it is sitting at, then speed up and slow down accordingly.
Apparently it is not looking at, or taking into account its position, to set it at the correct speed. So you cannot start in CSS from MDI to make a manual cut, or run from line with the spindle started correctly for the cut.