Earlier quoted context omitted.
There was a thread on dri-devel a year ago: https://lists.freedesktop.org/archives/dri-devel/2015-Octobe... 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…
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.
I guess I'll wait for Apple to actually announce new desktops before I seriously look into the driver situation...
[1] http://forums.macrumors.com/threads/bootcamp-update-5k-windo...