You're thinking it about it wrong.
Speakers create soundwaves that move away from a speaker in spheroid space, becoming increasingly on axis as frequencies rise. To offset this, the speaker diameter is reduced.
Mounting a speaker in a roof is almost Infinite baffle, so bass frequencies will emanate in a half spheroid space. Given that you have a nice small tweeter, these frequencies should also play into a half spheroid space as well. The problem with mounting speakers in the ceiling has far less to do with the directionality of the drive units themselves and far more to do with the pathlegth's of the listener to the speaker, and the walls/floor/ceiling.