The Arc uses two upward firing drivers to bounce audio off of the ceiling to your seating position to create the height channel effects from Dolby Atmos content. A high, vaulted ceiling will limit the ability to hear the height channels. A lower and flat ceiling is recommended for the Arc.

The virtual surround the Arc produces with its side firing drivers should work pretty well in that room though.

Thank you, GuitarSuperstar for your reply.  Although not at all ideal, I wonder will this be the “best of the worst” of the speaker options?  TV speaker is insufficient.  Do you have any other recommendations?  

I still think the Arc will sound great in your room. You just probably won’t hear the height channels when watching Dolby Atmos content.

The new Sonos Beam (Gen 2) is a Dolby Atmos sound bar that doesn’t rely on upward firing drivers. It is currently only available for pre-order, and no one has reviewed it yet to know how convincing the Dolby Atmos audio will be. It might be a good option for you.

https://www.sonos.com/en-us/shop/beam.html

This is a great idea, GuitarSuperstar.  I’m going to look into this now.  Thank you!  

Will adding 2 in ceiling speakers and amp upgrade the sound significantly or no?

If she had a flat ceiling, adding two in-ceiling speakers as rear surrounds to an Arc or Beam would give her a much more immersive home theater audio experience. But because she has vaulted ceilings, in-ceiling speakers as surrounds would be out of balance since one speaker will probably be installed significantly higher than the other. Trueplay tuning could adjust for some of the imbalance, but I’m not sure it would be enough.