Need to see if Apple has new drivers for that AMD new GPU card for the Mac OS. If Apple has drivers than your RX 590 should work. You can also see if the RX 580 Apple Driver is compatible with the new RX 590. If it isn't then you will need to wait until APPLE or someone else creates a driver for the RX 590.
These are the only Mac drivers AMD has : AMD Drivers and Support for Radeon, Radeon Pro, FirePro, APU, CPU, Ryzen, desktops, laptops
I bought an ASUS XG Station Pro e-GPU casing in combination with the Sapphire Nitro+ Radeon RX 590 Special Edition. The RX 590 simply runs.
It is seen by Mac OS Mojave as Radeon Polaris and as AMD Radeon RX Ellesmere Prototype. See screenshot attached.
Using this product with a Mac of any description for 3d gaming--unless it is used with Bootcamp support for properly configured Win10x64 installations--is wasting a good bit of the GPU's capability & power--because OS X does not support either the Vulkan API or the DX API (which are supported by the AMD drivers under Win10, etc.) . Additionally, last I looked, Apple's own version of OpenGL was quite a bit behind Windows, and only moderately tuned for gaming. (AMD now supports OpenGL 4.6 under Win10x64).
Additionally, as per the screen shot, the RX-590 is *not* a "prototype"--it's the finished product. Probably says "prototype" because of the immaturity of Mojave's support for it under OpenGL, I'm guessing. 50% the value of any GPU comes from the software that supports it. I suppose it would be fine for OS X if the majority of its use falls under OpenGL desktop--but in that event it is rather massive overkill just for desktop support.
If you want an RX-590 at all with a Mac--and you want it to run 3d-API applications and games--not just your accelerated OS X OpenGL desktop, etc,--highest/best use is with Bootcamp and Win10x64. Otherwise you may not come close to getting your money's worth. That's my advice, anyway...for what it's worth...;)
Thanks Waltc, didn't know that.
Hopefully Mojave support will improve and consequently the performance of the RX590 in my setup.
macOS Mojave: On Sept. 24th, 2018 Apple released macOS Mojave and posted a new support article Install macOS 10.14 Mojave on Mac Pro (Mid 2010 and Mid 2012), that mentions having macOS 10.13.6 installed first, (without FileVault enabled) and a list of compatible graphics cards, with a note about no startup video displayed from some 3rd party graphics cards (i.e. cards without a Mac ROM). Also, don't forget that firmware updates won't work if your startup disk is a RAID drive, so use an onboard SATA boot drive for updates/upgrades.
If you have more quarries then you can consult with Macbook Repair Singapore and get your answer.