If you want to run a 5k2k display over DisplayPort, it's reported working with a specific cable (see this post). I can't confirm for sure but it seems like DisplayPort connections are not affected by this issue. I assume that any 4k monitor connected ...
I don't know what the behavior of a 5K DisplayPort 1.4 display is on a Mac (or a PC). I have a dual cable 5K DisplayPort 1.2 display. I haven't seen info for an iMac 5K display (I think it must also be dual cable to support 5K 10 bpc)....