2010? The only way the lights will "CUT" the ceiling is if the architect places a fixture hosted to the ceiling (in the arch model). If you are MEP and "Linking" in the arch file, the lights will not cut the ceiling, even when you host them to it.

The -1/8" move down will show the fixture, but the light output will still be muffled by the ceiling (still there). If you only want to show the fixture in the rendering, the move will work, but you will have to modify every family type individually. Pick your poison.

In 2011, ceilings can be copy/monitored, so you basically have an actual "Ceiling" instead of a face, and it will be cut by your light. Diffusers should work in the same manner.