My room is fairly small with a 8.5' ceiling and debated between M55XC and M80XC. The M80XC's are less obtrusive than I expected. I wanted their lower bass response, so after mocking them up with a box the same size as the speakers, they seemed to be OK so I decided to go for it. They come with a short ball mount (InvisiBall) that is designed mainly for walls, I think. Mine needed to extend below my ceiling panels so required a longer drop. I bought some fairly cheap Pyle ceiling brackets and mounted the stock ball mounts to them. However, JBL makes a number of extended-stem ball mounts for ceiling mounting, so that would be my first choice. (When I made my plans I wanted to retain all my ceiling panels, requiring a much longer drop, but of course the speaker positions for Atmos were smack-dab in the middle of a couple of panels. I removed them and thus could have used the extended JBL mounts.) You'd need to double-check with a dealer to get the right ball size.
There are pictures of my system on ASR in one of the "about your system" threads.
HTH - Don