Sadly, I haven't heard/read anything good about running Linux on the iMac 5K, which I'm tempted to buy when/if it is updated in 2017. I don't understand Apple's vision at all, and it wouldn't surprise me if they ruined macOS in the next five years. Being able to install Fedora in that case would really buy me some peace of mind.
Back then the probed panel resolution (EDID retrieved by radeon) was 3840x2160, which doesn't make any sense because the panel actually has 5120x2880. It's unclear if this was because of missing/broken MST support in radeon (Multistream Transport) or if Apple used some proprietary mechanism to squeeze the panel resolution in a single stream transport. You may want to contact the person who started that thread (Andreas Tunek) and ask if the situation has improved with contemporary kernels.
If memory serves Apple didn't actually use MST for the 5k iMac and instead developed a custom timing controller which allowed them to get the 5k resolution working over a single stream.
It's the same reason why, for example, Windows sees it as a 3840x2160 panel as well.
