Exactly, I used this $7 cable from the link below to connect the W5700 to the DP-IN on the motherboard. From the motherboard to the monitor I'm using Apple's thunderbolt cable that came with the monitor. SwitchResX says 3008x1692 HiDPI @ 60hz. Everything looks great and sharp. Under Graphics/Displays in System Report it shows 6016 x 3384 Retina 6K. Presets all work. It's the most freakin' amazing monitor I've owned.
https://www.amazon.com/dp/B0786FNLDT/ref=cm_sw_em_r_mt_dp_QcfBFbA9MNQWQ
Use the EFI from Chris and choose the default config file with the iMac Pro SMbios.
Use the same settings in BIOS as Chris but enable everything under Thunderbolt (including thunderbolt boot or something like that) to get this working 100% with sleep and profiles after restart, and to get USB 3 speed for the hub on the back of the display.
If you need any other help let me know.
I'm about to try it with an inexpensive RX 580 tomorrow to see whether the monitor still works and I'll post.
Thank you very much for the info @gimarbazat . This is probably the first time this has been tried.
I have RX580 and based on what @joevt is saying, I am gonna need to buy a W5700 blue card. Do you know if the cheapter RX 5700 XT work suffice? They seem to be the same card. There is one locally available for pick up so I could just go buy 5700XT + haul in a giant white box from the Apple store for $5k and have a grand weekend
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