I'm considering getting 2 era 300 as rear/atmos for my arc and sub.
I can only place them on the side walls above listening position. Any advice on exact placement.
One thing that might be a gamechanger is my acoustic ceiling, which is more or less designed to stop sound waves. Will an upside down placement be better in this case?
Yes, you will probably be better off mounting the speakers upside down. Check out this user guide to see how to install the speakers using the wall mount:
https://www.sonos.com/en-us/guides/era300#to-era-300-wall-mount
Just be aware that my set up necessitated wall mounts with my play1s as they were on the side walls. However when I upgraded to Era300 there is insufficient angular swivel on the Era300 wall mounts to point them in the correct direction 
I'm considering getting 2 era 300 as rear/atmos for my arc and sub.
I can only place them on the side walls above listening position. Any advice on exact placement.
One thing that might be a gamechanger is my acoustic ceiling, which is more or less designed to stop sound waves. Will an upside down placement be better in
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