Update 23.01.2020: I have confirmed that the Asus PA32UCX, UltraStudio 4K Mini and Final Cut Pro do indeed work with HDR PQ.
The reason to use a dedicated IO card (like the UltraStudio 4K) is that it gives you a properly managed colour pipeline that by-passes the operating system’s GPU and colour profile settings and gets you straight from the NLE to your monitor without alternation (unless you’ve got some hardware calibration going on too). – Jonny Elwyn
Up until this afternoon, I thought that was the general consensus, but apparently not! There’s been a lot of ink spilled by dozens of recognized authorities as to whether or not an I/O capture card is even required depending on the NLE, and which devices will work and which will not. So I was understandably troubled when I read this quote attrubuted to Patrick Southern over at Lumaforge:
You have to use an AJA IO 4K or IO 4K Plus with Final Cut Pro X to output proper HDR. You’ll only get an 8-bit image with a Blackmagic box out of FCPX, and you’ll only get the proper HDR metadata to the display using AJA. Resolve, on the other hand, must use one of the Blackmagic IO boxes to properly output HDR.
While the article may be outdated (1-1/2 years is an eternity in the tech world) I was still worried, since I picked up the UltraStudio 4K Mini last week in the hope of dabbling with HLG in Final Cut Pro (my monitor arrives the end of December). I shot off an email to Ed Tan from Blackmagic Design in Singapore, who sent me a reply from Resolve specialist Zach He, which I am sharing here.
For HDR output, our Ultrastudio Mini 4K and Extreme 3, is definitely able to output 10bit with FCPX. I’m not too sure which ‘Blackmagic box’ Patrick Southern was talking about, and when that quote was made by Lumaforge (can’t find the original article from lumaforge), it could be very dated. But just so everything is clear, when paired with Final Cut Pro X, you are able to output up to 10-bit RGB 4:4:4 in HDR (Rec 2020 PQ or HLG). That goes for both SDI and HDMI connections. This will work fine with the ASUS PA30UCX, of course we always prefer If they could use DaVinci Resolve 🙂
He then supplied a screenshot for proof.
The Daejeon Chronicles 
I’m confused about this too. If I plug a UHD TV into my Mac Pro’s HDMI port will Final Cut Pro be able to use that as an HDR display? Will my Rec.2020 HLG movies play in HDR on the TV, or do I have to go through a Thunderbolt I/O device?
Unfortunately, HDMI 1.4 does not support HDR. You will need an I/O device for accurate color. The 2013 Mac Pro is connected via Thunderbolt to an I/O box like the UltraStudio 4K and from there via HDMI 2.0 to the HDR TV. That way, the signal goes straight from the software to the monitor and by-passes the graphics card and MacOS color management.