Yup, after doing a bit of research on this, I came to the same conclusion. Thanks.
The good news is that I have a Mid-2010 Mac Pro (5,1), which I also just upgraded to High Sierra. I popped in that cheap RX 560 that I bought for my CustomMac (HP 8300 SFF) and it too worked like a charm under High Sierra with the Mac Pro 5,1. So if anyone has a Mac Pro 5,1 that's a bit long in the tooth, and they want 4k, that card is cheap and works great.
Yes, with that card I have the same issue on my Mac Pro with there only being one DisplayPort output and thus, the second monitor is HDMI @ 30hz only and has non-accurate color, but the main monitor is 4k @ 60Hz with my Mid-2010 Mac Pro 5,1. It really feels like a new computer.
I plan on getting a Sapphire Pulse RX580 for my ground up hackintosh, and while decide on which parts to get, I can order the RX580 card first and place it into my Mid-2010 Mac Pro 5,1 for dual DisplayPort glory. Just ordered a
Mac Pro Dual 6 pin to 8 pin power cable as well as this will be needed, and that will buy me some time to keep this Mac Pro going with the dual 4k displays while I create my replacement hackintosh. All great info, and I really would not have known all of this stuff without first dipping my toe into the hackintosh world with your HP 8300 guide,
@trs96 so thanks again.