You’re on the Sonos forum, so get the Arc and two Sonos speakers to be positioned behind the preferred viewing position - this does not seem possible with the built in speakers (they are in front of the seats where the remote lies). My only concern is with you viewing position; it seems a bit far from the TV and the soundbar - maybe it is just the photo. 

What problem are you attempting to solve here?  You said you already had atmos compatible stereo, so what are you hoping to accomplish by getting an Arc?

I don’t know why you would go from physical ceiling speakers and what I’m guessing is an Atmos capable Amp, to the attempted reflected Atmos effect with the Arc.

I did the opposite and went from an Arc to a Marantz theatre system.

If I go all sonos, then

Arc plus sub plus surrounds… options could be an amp to the rear in ceiling speakers versus a play or would wall mounted speakers be better for atmos surround?

Why did you do that and how do you compare?

trying to get the best dolby Atmos sound for my home theater,and

using some of the speakers for music when not watching TV, and

making the best use use of four pre-existing ceiling speakers

not that far away…. telescopic view to capture everything….  82” tv is about 12’ from the chairs

Thanks for all advice and opinions

I didn’t get the immersive experience I was expecting out of the box.

I watched Avengers Infinity War and found the listening experience unsustainable.  Clarity of sound etc but too much of an assault on my ears.

Although the soundstage was better than my tv speakers, I didn’t get the Atmos or any surround effect.

I faced a choice: Wait for Sonos to improve the highs and mids and spend another £1000 on sub and surrounds, or build a ‘proper’ theatre system.