@woland You could also look at the Blackmagic cleanswitch, it is a bit of a tricky beast as it will work better with reference (or reference SDI input 1). This would give you a clean switching matrix that could be more useful than a simple mixer. I am pretty sure it also uses UDP, if Izzy gets UDP (it's been knocking around on feature requests for a while) then you could control it within Izzy.
Above you can see it capturing an output from isadora running at 1920*1080 and up-scaling it to 3840*2160 (see below). I have since read it can capture a single HDMI at full 4k - which I will try one day.
The hardware cost £435 which is pretty cool as this can capture DUAL HDMI inputs. So its almost £200 a channel input capture.
Frame delay is minimal but the quality is amazing.
PCI Express Gen 2 x 4 low profile
And it is indeed low profile! I was very tight on space.
I have done now hardcore testing yet but it works and I will keep you posted on my research.