Hi! I have an iMac 2020 8-core i7 with Radeon 5500XT GPU.
I don't know yet about the performance since I've been running into issues with the GPU and the HDCP handshaking resulting in a stuttering video output on all screens every 2-3 seconds.
There are a lot of threads on the Apple forum about it but no fixes has been done yet.
https://discussions.apple.com/thread/251766223
This means when I connect a non-HDCP device to my iMac it stutters the video output.
Same is happening with the 5500M GPU (all threads are about the 5500M).
I will do some performance testing with 2x 4k output in the future and will report back. For the iMac that's the maximum amount of displays as I know of.
I know the 5500XT is not the very same as the 5500M but they are kinda the same I guess?
Update: I tried several video adapters, they were all stuttering (including the Apple multiport AV adapter and others from the brand DELOCK). Then I got my hands on a dutch branded adapter called ALLTEQ USB C TO HDMI - HQ KABEL (allekabels.nl) and those cables brought me 4k60 (the others maxed out at 4k30) AND they stopped the stuttering while connecting to a DECIMATOR 12G CROSS. I was speechless.