Thank you for experimenting and testing! I'm still not sure what to do with the "Use 4:4:4 SDI"-option. As I mentioned it behaves the same (increases brightness a bit), regardless of what I set the SDI Output in my UltraStudio settings, which makes sense, since I use the HDMI-output.
I've set the monitor to RGB, Matrix auto and and the input range to full; Resolve is set to full, too. I generated an SMTPE chart within Resolve's generators and experimented with the "Use 4:4:4 SDI"-setting.
Switching "Use 4:4:4 SDI" on makes the image a bit brighter and makes two small bars on the bottom of my SMTPE chart visible. Looking a my scopes, I should indeed be seeing these two bars, since they are clearly elevated from 0 IRE... turning "Use 4:4:4 SDI" off makes the two bars invisible... this happen in full/full and video/limited.
I guess I'll probably leave "Use 4:4:4-SDI" on for the moment...