​@HTG 

First of all the audio quality of each audio track depends on the original upmix. And there are huge differences in mixing quality. Some movie audio tracks are very poor even if the format is DD5.1 or Atmos. 
Second fact is that the best stand alone soundbar setup can’t do the same (especially regarding to rear channels) as real dedicated speakers. 
Arc Ultra just can play virtual rear channels. Also some of Arc Ultras chassis that primarily are used for Atmos signals (hight tweeters, wide tweeters) can’t impress that much with a DD5.1 signal. 
So imo Arc Ultra on its own is limited in  playing DD5.1 audio compared to a real 5.1 system including dedicated rear speakers. 

Your question mentions “side speakers”, ​@HTG  You can’t have them. The Arc Ultra contains the three front channel speakers. You can add rear speakers to that, but not side speakers. 
 

Sorry, though: I can’t comment on whether adding them would improve the 5.1 experience as I don’t have an Ultra. 

Maybe terminology sometimes isn’t quite exactly and that leads to misunderstandings. 
To be exactly depending on the audio format there are side rear speakers and back rear speakers (and also back hight speakers). 
So sometimes Users might call it different from what they are talking about. 

Ultra tries to give you a Dolby/Atmos experience when playing such a format. It succeeds quite reasonably but will never be optimal without surround speakers and preferably a Sub as well.

It will be a bit technical to explain but imagine sound that should come from the side or behind you coming in front of you, not optimal.
However, Ultra tries to spread the sound with all its speakers, up to the side and front. It tries to shoot the sound diagonally up so that it hits the ceiling and then bounces down behind you to give you an experience of the sound coming from behind you. Again, an excellent attempt, but only an attempt.

You will experience a completely different sound universe with surround speakers, in my opinion a must have.

Thanks for the interesting feedback folks! It sounds like I need to at least experiment with rear** speakers as my next step, when budget allows. 

** Oddly enough, I did understand that Arc Ultra needs rear speakers to complete the ensemble when I first posted. It’s a telling slip, that I said “side”because that’s the sound zone - that is, in the physical space based on my current setup - that seems lacking. But now hearing you all, it’s likely the rear tracks that’s more the problem. So there’s the direction I’ll try to head! 
 

Thanks again! 
 

Edited for clarity. 

​@HTG 

If budget and room situation allow, I recommend a pair of Era300 for best multichannel experience. 
But also a pair of Era100 imo will do a great job. 

​@Schlumpf Good to know! Space is definitely also a factor, so knowing the smaller ones won’t be a total bust… 

Era 100 are really good surround speakers, without a doubt.
But Era 300 has far more speakers such as sides and upwards, it will spread the sound better.

Era 100 only has front speakers, so the sound will be more concentrated/evenly directed. Where Era 300 will have its front speakers deactivated when it is set up as surround so the sound will come from the sides and upwards.

I myself have Arc Ultra-Sub 4-Era 300. It is a pure pleasure to put on a movie or listen to music.