Hi

The main issue I see is that the Arc may block the lower portion of the screen (the same would be true with the Beam). If you can live with that running Trueplay in the Sonos app would most likely adjust the sonics for the less than adequate speaking for the side speaker reflections.

Adding Sonos surrounds and a Sonos sub will enhance the overall sound. The 8 ft ceiling height is not a problem. Quite frankly it’s a plus (especially if it’s a flat non-textured surface). 

Your next decision is whether select an Arc or Beam in black or white. Personally, I always chose black because it mutes the visual presence of the sound bar which IMO should be “heard and not seen”.   But then again...given the cabinet color white may be the better choice.

Thank you. I had planned to raise the TV on a small shelf and push it a few inches back to make room for the soundbar. 
I am planning on getting a sub, but not sure about surrounds. The sectional sofa  in the room backs up into the corner so I’m not sure if there is a good location for one of the surround speakers other than directly above the listener’s head. 

For the ceiling height, I was worried the reflected sound wouldn’t reach far enough back in the room, depending on the angle the upfiring speakers are pointed. 

totally agree on color. I was leaning white based on the rest of the room. 

Did you consider mounting the Arco on the front of the cabinet below the TV space using the wall mount?

https://www.sonos.com/en-us/shop/wall-mount-for-arc

That’s an interesting idea. I hadn’t seen that, but am a little concerned about mounting something in the front since my 7-year old plays in that room sometimes (you can see the Hot Wheel tracks stored in the cabinet).