I do have the same setup as you have, except I don’t have a Beam but a Playbar. I also tinkered with the DSP to equalize the room like explained elsewhere on the forum.
I don’t know whether it is a limitation of the current firmware or of the hardware, but the Sonos components are only able to stream and play stereo music. For the Beam/Playbar/Base there is one exception: they can accept 5.1 signals on the optical or HDMI input. Therefore Roon sends a stereo signal that is upmixed by the Beam. I don’t know what algorithm is used, but a surround mix is created from a stereo signal by the Beam. Every effect the DSP has on the sound gets upmixed as well, so will also be applied somehow to the surround channels. However, as Roon sends only a stereo signal, there is no way the DSP can influence the surround effect (unless maybe someone is aware how exactly the algorithm works; cf Dolby Pro Logic)
It’s perfectly possible to enhance the sound, as I’ve tried to do, but be aware that this will influence the sound of the Beam as well as the surround speakers at the same time. (And they may not need the same correction.)