How far will you be seated from the Arc and Era 300s? Is your ceiling flat?

Yes our ceiling is flat but V-groove panelling.

I think I would get the Era 300s and return the Amp and in-ceiling speakers. But I would still run speaker wire in the ceiling so you still have that option in the future.

What about simply adding the ERA 300s? 

Or would they just make the in-ceiling obsolete.

If they don’t accept the refund (I bought them a while ago).

Thanks for your views!

You can only add two surround speakers to a Sonos home theater setup. So you have to choose one or the other.

If you use the Era 300s as surrounds with the Arc, you could group the in-ceiling speakers when streaming music. But grouping them while playing TV audio from the Arc will result in a delay which creates an echo effect.

You might consider installing the in-ceiling speakers above the dining area to use as a separate zone for music.

Ahh ok - Thanks!!

I was using the arc, sub gen 3 and two amp powered ceiling speakers (as rears even though they are positioned almost directly above our couch) and was extremely happy with the set-up. 
 

I then got the itch to buy era 300’s and eventually succumb. I now have the 300’s set up as rears and grouped the ceiling speakers as a stereo pair to work in conjunction with the arc, 300’s and sub. All I can say is WOW. The sound is so full and satisfying. Do not hesitate to use 300’s as rears in conjunction with your ceiling speakers - you won’t be disappointed.

Also, there is ZERO echo or delay - everything is perfectly in sync. 

There's a delay, you can't avoid it, it's built into the design. I know some don't think they hear it or wish it away, but the fact remains, there is a delay.  Also, those grouped speakers will be playing all the channels of surround sound, front left, center, right, surround left, right, and height left, right all right above your head.  This will destroy any directional cues and make the surrounds a mish mash of all channels, delayed by 70ms.  More speakers does not mean better sound.

Do you have the system I referenced? I do, there is zero perceivable delay. 
 

As for the grouped stereo speakers playing all channels - yes that is correct. I have those particular speakers set at a lower volume (14 units lower for my use) and I can confirm they do nothing but enhance the sound despite playing all channels and there is no loss of overall ‘surround’ experience (quite the opposite in-fact). 

If you increase the volume of the grouped speakers so they match the volume of the Arc, do you hear a delay or echo effect?

If rear speakers are “Bonded” as surrounds, they will be synchronized with the front speakers and become a single surround Room. If the ceiling speakers or ERA 300’s are simply installed as a separate “Room”, there will be a time skew between TV sound and output from any “Grouped” Rooms. The Grouped Rooms receive stereo sound, not surround sound. When playing music, all members of a Group are time aligned.

Some listeners are very sensitive to any time skews, other listeners are not. Consider yourself very fortunate if you are not overly sensitive to time skews. The time skew police spend enormous effort complaining about this or that, rather than enjoying. Some time skews are built into the production by sloppy producers or content delivery platforms, others are due to the physics of sound propagation in the listening area.

That’s a fair assessment. If I could hear an echo I would turn off the stereo pair immediately. To me, it sounds better with them on.