Referring to overhead speakers as "atmos" is a misnomer as the term does not pertain to any particular speaker location. Rather, the Dolby Atmos codec is just one of the immersive surround-sound formats (in addition to DTS:X and Auro-3D). Nonetheless, those overhead speakers in your setup will most likely be used for a lot more than just audio objects; in fact, at this point in time the selection of content authored in Dolby Atmos or DTS:X is minimal compared to the non-immersive sound tracks (and you can all but forget the misbegotten Auro-3D, which never took off here in the U.S.). Therefore, if your usage is typical, you will more frequently be sending upmixed channel signals to those speakers.
HST, Dolby themselves recommend the use of full-range speakers for the overheads (see below). While you may take their recommendations with a grain of salt, I myself would not treat the overheads any differently from the rest of my surround speakers regarding their capabilities.
"Dolby Atmos audio is mixed using discrete, full-range audio objects that may move around anywhere in three-dimensional space. With this in mind, overhead speakers should complement the frequency response, output, and power-handling capabilities of the listener-level speakers. Choose overhead speakers that are timbre matched as closely as possible to the primary listener-level speakers. Overhead speakers with a wide dispersion pattern are desirable for use in a Dolby Atmos system. This will ensure the closest replication of the cinematic environment, where overhead speakers are placed high above the listeners."
Dolby Atmos® Home Theater Installation Guidelines
One major reason why in-ceiling speakers are preferred is that effective three-dimensional imaging requires the overhead speakers to be mounted at a certain minimal elevation that is difficult to achieve with the low ceilings in most home environments that lack a custom-built theater. If your ceiling height is eight feet or less, mounting the overhead speakers on the ceiling (or on the walls) is apt to result in too low an elevation angle due to the depth of the speaker assembly in addition to that of the mounts. The elevation of the overhead speakers relative to that of the listener- (ear-)level speakers creates that dome of sound, as you called it. The less the separation between the two levels, the more the dome collapses into a mere two-dimensional effect.