So if Ian told you the M3c's don't image well do to their recessed driver are you going to use the in-wall M3's?
I'm still trying to figure this out for myself.
Here are a few links you might be interested in. The first link talks about how important it is to have wide dispersion in-ceiling speakers.http://www.acousticfrontiers.com/dolby-atmos-dispersion-requirements-for-ceiling-speakers/http://www.acousticfrontiers.com/ten-speaker-layout-tips-for-dolby-atmos-dts-x-auro/
If I assume that the post your referring to is mine (I decided not to use the M3c for my Atmos ceiling because of concerns with imaging). My thinking:
Atmos is an object placement format. The movies sound engineer states “this object is at this point in 3D space and here’s it’s audio track”. Given that infomation each play back system (yours, mine, your local movie theater, etc) figures out how to get it there by using its unique speaker layout and locations. How close to the exact location will depend partially on how well those speakers image.
That said, at the moment Ian is probably right ... most of the time it’s just an overall effect, the exact placement does not matter. But the power of Atmos is when is does matter and I suspect as time goes on it will matter more and more as engineers embrace this format.