Will new Sonos ARC provide full featured Dolby Atmos quality sound when connected to: 1) ARC-enabled Samsung TV (Q9; late 2018 edition); and 2) latest Atmos-enabled 4K Apple TV?

Please define “full” and “limited”? 

Well I am not competent enough on the subject to give you precise audiophile definitions … of course none is none and that would mean I just stay with the Playbar and forgo the upgrade to the Arc (not replacing an 18 month old high end expensive TV at this point) … but “limited” relates to more expert insights by some on this site (and elsewhere) who have said that you can use the Arc with ARC capability even if you don't have eARC but that would mean you will get less than the fulsome quality and features that you would get with eARC capability … that help clarify? Thanks.

Sorry I meant to add that “full” means the full Dolby Atmos immersive surround sound, voice sync, all speaker utilization.

Further notes: right now I see that my Samsung Q9 TV is set to “PCM” … and in Expert Settings Dolby Digital and Dolby Digital Plus are greyed out in the current setting and connections (optical right now not ARC).

Although it varies per TV to what extent Atmos is passed, the most you can get with ARC is Dolby Digital Plus with Atmos.  DD+ is an enhancement over standard DD, but it is still lossy.  To get TrueHD with Atmos, which is lossless, you need eARC.  Note, most streaming services use DD+, there are not many sources of TrueHD outside of Blue-ray.

So, you could define, if you so choose, the two kinds of Dolby Atmos based on the bandwidth required to carry the data. “Limited”, to use your term, is carried on Dolby Digital Plus connections, which require HDMI ARC or higher bandwidth connections. It can not be carried on optical connections.

“Full” requires the use of Dolby TrueHD, which is carried on eARC. This will carry all available speaker channels to a full Dolby Atmos system.

Caveat here, I don’t have an Arc yet, but I’d be stunned if you could hear the difference between the two kinds of connections types when using a Sonos Arc. It’s a sound bar. There are built in limitations about numbers of speakers and speaker placement. I suspect that Dolby Digital Plus, via HDMI ARC will be quite satisfactory for using the Sonos Arc. However, if you have a 20 speaker + sound system set up with all the required speakers in the correct locations/orientations, I suspect that you could hear a difference in the sound between the two formats. 

So, with that information imparted, I’ll suggest these answers to your questions:

1. Limited, as defined by the connection type, assuming your TV can pass Dolby Digital Plus via HDMI ARC. 
2. No one is sure yet. There’s discussion around that Apple uses the the MAT format, which is only carried by eARC, but there are some TVs that might be able to translate that to HDMI ARC. Until you have an Arc and can test, there’s no real great way I’ve seen to figure this out. Looking a rtings.com is probably the best at this point, but I’m not convinced that I’m comfortable with the accuracy, having not done any of this testing myself. They could be 100% right, and I lean that way, but I just am unwilling to put money on it. 
3. Not currently. As has been discussed in many, many threads, everyone is looking for a device that splits off an eARC signal from an HDMI input and sends that out. No one has found one yet. 

Thank you. As a laymen in these matter I take what you are saying to be encouraging. I don’t use Blue-ray any more -- our family streams via AppleTV+, Netflix, Disney+, and Amazon Prime Video.

So if I understand what you are saying does this mean if I switch the connections from the optical hook up to the ARC one and attach that to the new Arc bar when it arrives I should be able to get the full benefits DD+ with Atmos with those streaming sources. That make sense?

Yes, except for maybe Apple TV. That remains to be seen. I’m anxious to get my own Arc, and see how the Vizio handles the MAT signal from the Apple TV to the HDMI ARC port.

I’d like to by a Sonos Arc for my Apple TV 4K.  Apple makes it clear on their site that ATV does not play Atmos via Arc.  So it looks like I won’t be buying a Sonos Arc.  Unless after the Arc is released it magically works with my ATV and plays Dolby Atmos.  Pretty sure that won’t happen.

I basically have the same setup as you (2018 Q9F, Apple TV 4K). From everything that I’ve read, I will not be able to pass an ATMOS signal to the Arc. As you note, the Samsung does not have eARC, so “full” ATMOS was never going to happen - I don’t even have a Blu Ray player so this wasn’t a realistic option for me as I understand streaming services do not carry this. 
 

For “partial” ATMOS, my understanding is that the Apple TV will not pass the signal via DD+ like some of their competitors (Roku, for one). I read it is actually a higher quality version than the lesser implementations within other streamers, but is too big to carry over ARC. So, it is likely that this setup will not give “partial” ATMOS either (Apple recommends connecting directly to soundbar, which obviously the Sonos ARC is not capable of). This is of course all speculation until someone can actually test it. 

I will be interested to see confirmation of the above, and similarly if getting the benefit of Dolby Digital+ alone is worth upgrading from my Playbase. Also curious to hear feedback in music / separation. I’m happy with the Playbase in my condo, but could always be happier.