As previously mentioned, your setup is the problem, not the number of channels. Surrounds aren't intended to be firing behind the listening position, but to the sides. The benefits 7.1 can be realized if your seating position is a significant distance from the back wall. It would be helpful if you post a picture or sketch up of the listening position and position of the speakers. If the in ceiling speakers are truly behind the listening position, it might be beneficial to use them as surround backs in conjunction with surrounds firing to the sides. Is there a reason you are set on using in ceiling or in wall speakers? It's sounds like this is a dedicated theater room so living room aesthetics shouldn't be a factor. Wall mounting would be a lot less of a hassle and sound a whole lot better. Ideally both the surrounds and surround backs should be placed on wall or in wall 2' above the listening position at 90-110° and 130°- 150° respectively. It's okay to place the surrounds higher so long as the speakers are angled towards the listening position. Considering you've already got a 9.2 receiver, if the current in ceiling speakers are overhead of the listening position, I would reassign the in ceiling speakers for top middle Atmos duty. Myself personally I would avoid using in ceiling speakers for any channels in a classic 5.1 or 7.1 set up, height channels aside, none of the sounds in the L/R surrounds or surround backs were intended to come from overhead. The use of in ceiling speakers for these channels is an aesthetic/space compromise, and a poor one at that.
Assuming the room is large enough and the MLP is at the proper distance, it's always worth it to add more channels to a surround setup. Money and practically (which shouldn't be a factor in a dedicated room) are the only reasons to not add more channels.
Just from my experience with different channel configurations, I would first go with 5.1, then 7.1 utilizing heights or front wides, and finally 9.1 adding in surround backs. Rear surrounds contribute the least to the sound field. In my opinion, with multiple speaker arrangements to choose from going beyond your typical 5.1/7.1 configurations these days, they should be the last addition to your setup. If your receiver supports a front width configuration I would completely forget about rear surrounds and simply go with a wide, height, and surround configuration, placing the surrounds at 110°.