My problem with having surround side speakers so low is that they blast in the far most seating position listeners ears while the center most seating position listener now gets an obstructed sound path from other listeners in the way. With surround side channel speakers above everyone's heads, the sound isn't blasting in anyone's ears and everyone gets an unobstructed sound path for clean sound.
I originally had my surround and surround back speakers on speaker stands (donated to good will), but when I got married, I had kids and more animals. The speakers didn't stay standing all the time so up high on the walls they went. What a difference, they sounded so much better. Even though Dolby recommended they go up high, I initially was hesitant to scar my walls with holes to mount speakers. After getting married, I found if I wanted my speakers to last, they needed to be on the walls. Now the kids are older and I'm down in the number of animals (unfortunately) but the remaining still ensure the speakers stay on the walls.
Due to furniture now, I couldn't bring the surrounds to ear level but maybe 1.5' above ear level. Still above the head of listeners. However now this leaves 1.5' between back surround height and surround height. Not sure that's enough.
I still have 1' from the top of the surround back speaker to the ceiling but then sound anamolies could happen being it's where the wall meets to the ceiling.
So all the way to the top, I'd have 2.5' (center of speaker to center of speaker) between surround speakers and surround back speakers, but back speaker's sound could be compromised being where the wall meets the ceiling. This is with 8' ceiling.
I wouldn't ever buy a 9.1 receiver as I just don't want that many speakers all over and don't want to waste money on amps that wouldn't be used.
So the issue would then be I'd have to stay with Onkyo as of right now, Denon and Yamaha do not offer a 5.1.2 configuration with rear height channels. You'd have to step up to their 9.1 receivers.
As of right now, I may just keep the 7.1 setup I have. Maybe I'll change my mind but Atmos height speaker use seems too demanding on the room's furnishing layout. Since my listening position is close to the rear wall, dolby enabled rear speakers wouldn't work as they would reflect into center of the room.