Thanks for the reply, the two modes do look much closer in the group view once I changed all the settings and limited group FPS to 5 instead of 10. And in the single camera mode, the 2160p is definitely much nicer looking. I had hidden the PTZ controls on the app a long time ago so I had actually never seen those group options until today!@polldav
First, make sure your group frame size is decent here. By default it only comes out at around 720p resolution which sucks on a large display.
For a long time, Blue Iris always streamed H.264 camera groups at their native frame size (as set in the location above). That changed a few months ago so that resolution limits in streaming profiles can cause camera groups to be streamed at a lower resolution.
Jpeg HD is a much simpler but less efficient streaming method. UI3's 2160p profile should be the same in terms of image quality and deliver a better frame rate with less bandwidth usage.
I think essentially the 2160 must prioritize FPS and the Jpeg views prioritizes the full image size (now that I unchecked the frame size limit). For my group stream that was comparing a 1.5fps Jpeg to a 10fps video feed. Now that I limited the group to 5fps the 2160p setting looks a lot nicer.
Edit: this is all over a LAN