Sonos Arc Mount will fit the Arc Ultra, so there is a good chance that your Sanus mount will as well.  You could try contacting Sanus with that question, but they may not know the answer yet.  If you ordered already, you may be the first to know for sure.

Sanus had no idea lol

Unsurprising, honestly, as the Sonos Arc Ultra was released today, and the CS for Sanus probably is completely unaware of it. 

Danny, is there an official statement that Arcs wall mount (the original Sonos one) will also fit with Arc Ultra?

I‘m wondering because Sonos lists two separate products on their store and the two wall mount differ in weight though they look identical on the first look. 

Unknown. I chatted with Sonos and called tech support and got no answer. Chatted with Sanus and was asked to email their tech support 2 days ago and have not heard back. 
One would think that if Sonos actually cared about a customer’s situation they would have considered this obvious question and had an answer ready. 

Sonos is not responsible for 3rd party compatibility. But for their own mounts they say „Arc wall mount is compatible with Arc Ultra, but you might see parts of the mount because of minimal differences in design between Arv and Arc Ultra. If you like to get it perfect you better should use Sonos wall mount for Arc Ultra. 

I have a slightly different take on this question. I custom made a stand with holes drilled for the Arc. The dimensions listed on the two products surprise me (see below). For one, except for the width, the Ultra is smaller. Can anyone tell me where the mounting holes are on the ARC Ultra? Are they located the same distance apart as the the ARC?

Arc Ultra

M5 x 0.8mm thread pitch L6.5mm

Height: 2.95 in (75 mm)

Width: 46.38 in (1,178 mm)

Depth: 4.35 in (110.6 mm)

Arc

Threaded mount 0.5 in. (12.7 mm) / 10-32 threaded mounting socket

Height: 3.43 in (87 mm)
Width: 44.96 in (1,142 mm)
Depth: 4.57 in (116 mm)

@Cloxo 

I think they have to be… otherwise I could not explain how Sonos made their own mount for Arc compatible to Arc Ultra.  

For the record, it does fit! Got my preorder today! 

Congrats… and thank you for the feedback. 

Could you share a photo showing this part of the mount that doesn’t perfectly align with the new speaker ?

Greg