Could I use the M2 as the rear instead of using a 30 degree angled in ceiling for the 5.4.6 Atmos? Since the rear height has to be angled anyway I would think that work. Has anyone tried it?

Absolutely, in fact since I could never be bothered going through the hassle of doing a permanent install of ceiling speakers in my 7.2.4 system, although not M2s, I have 2 similar style speakers angled downward for each of the front and the back. This is really no different than going out and buying the specialty speakers from either SVS with their "prime elevation" models or Klipsch that are used in the same manner but go flush against the wall with the front of the cabinet being angled downward, same effect, and depending on the movie, gives a very close representation of the effects obtained from in-ceiling speakers.

Thanks for the input. I will still create the cavity in the ceiling in the event I decided to change to in ceiling down the road but I figured since I have the m2s coming anyway it might be a good use for them given their small footprint and they will be easy to angle and adjust using the fmcb.