To be honest is not so bad, is little lag but resolution is 4k even is only 31mb loaded.
I found it unusable. Just so slow and laggy. Any time you move your mouse pointer over something that it wants to accelerate - like the Dock for example - there's a really annoying pause. I couldn't live with it myself.
I've got my X299 system set up with two GPUs. The 6800 in PCIe slot 1, and my older Vega 64 in PCIe 3. I'm using an SSDT to disable the RX6800 in macOS, and just using the Vega 64 in that OS. Then when I boot into Windows, it's the opposite - I use the RX6800, and I've disabled the Vega 64 via Device Manager -> Disable Driver. The latter isn't strictly necessary, I could have the Vega 64 enabled as well in Windows, but I don't need it and didn't want to risk any games getting confused by the two GPUs.
This config is working OK for me, except sleep is broken. I believe that's because I haven't yet also disabled the USB hub on the 6800.
That's something rather interesting about the 6800 cards - at least the reference ones: they have a USB-C port. And it literally is a fully working USB port! I tested it out yesterday: I used a USB-C to USB-A adapter and plugged in a USB flash drive into my GPU, as weird as that sounds, and it was detected and fully working in macOS.
I can't imagine anyone needing or wanting to use their GPU as another USB hub, but if you have a reference card that is now possible. Of course it's meant for VR headsets.
It does make me wonder whether there might be any weird compatibility issues using the reference 6800s in macOS once drivers are supported. I hope that USB-C port works normally for connecting a monitor to, as I need to use all four display outputs on the card (and I wish it had more.) It's supposed to work fine and does in Windows, but I could imagine that could be a potential source of incompatibility, at least on hackintoshes - whether the USB to DisplayPort conversion works out of the box or if we will have to mess about with anything to get it to work.
I'd have preferred to get a card that didn't have the USB-C, and had 3 x DP + 1 x HDMI instead. But AIB availability has been terrible in the UK, so I bought the reference 6800 when I had the chance rather than have no new card at all.
Anyway, fingers crossed that in early 2021 we finally get some working drivers to try out.