Rx580 with boot screen is achievable.
I have apple boot screen and boot from Nvme M.2 SSD 1tb SiliconPower also. To get boot screen you have to "Create Install Media"
It's a command applied with terminal to give you a bootable installer.
https://support.apple.com/en-us/HT201372 .
After playing with a few setups i will tell you this. installing only one 6 pin power to the RX580 will allow you to use the card, however, the card will only run on "low power mode". The 6 pin connector senses when pin two is missing and throws the Rx580 into a safe power mode. The card knows, not to overdraw power, from the connector it's connected to. Its a fail safe connection that was designed into the Power connector itself. Having the 6 pin will "Not" allow you to put the operating system to sleep.
6 pin = no sleep
8 pin = No problems
Hdmi out vs DisplayPort output.
I have a 49" Spectre Ultra High Definition Display. Wall Mart Special.
I have achieved 5k Retina quality with the 4k Display. Want to know how ?
You have to use the Hdmi output on the Rx580 in order to get the 60hz function to activate in display preferences.
If you plug into the DisplayPort output you will be locked out from 60hz from within the operating system.
The display port pinouts and the software work together to allow the system to properly handle the data.
The software knows when a displayport cable is connected and adjusts the settings within the operating system.
View attachment 845458 Hdmi = 60hz output @ 5k (HiPDI)
Displayport = 30hz output @ 5k (HiDPI)
Under system preferences >Displays>hover mouse pointer over the "scale" button and press "option"
This will give you HiDPI options to choose from. This is a configuration that links your hardware with your software. It has to be manually set for your own preference. A Hdmi 2.0 cable is needed to activate 60hz function. The HDMI 2.0 cable must be in the Rx580's Hdmi port also.
I choose 1920x1080 HiDPI. Not 1080p or 1080i. This gives me a 16:9 resolution @ 5k. I only notice one application that has a latency lag with the mouse pointer.
Propelerhead Reason. Also Note: You won't see the HiDPI lettering in Mojave to let you know it's a HiDPI option. But it is, trust me. When I use Propellerhead Reason i go back to 1920x1080p @60hz. This Lower resolution allows me to handle (click and grab with the mouse pointer) at incredible speed. 5k@60hz there is a noticeable delay when i try to drag and resize windows, but only with Reason.
Here is my system currently:
Mojave:
10.14.5 (18F132)
Mac Pro (Early 2009) (5.1 firmware flash)
2 x 3.46 GHz 6-Core Intel Xeon (12 core w/ multicore hyper threading)
48 GB 1333 MHz DDR3 (slots 4 & 8 empty. Enables Triple Channel Architecture)
Radeon RX 580 8 GB Saphire Pulse
Radeon RX 580:
Name: ATY,AMD,RadeonFramebuffer
Type: Display Controller
Driver Installed: Yes
MSI: Yes
Bus: PCI
Slot: Slot-1
Vendor ID: 0x1002
Device ID: 0x67df
Subsystem Vendor ID: 0x1da2
Subsystem ID: 0xe387
Revision ID: 0x00e7
Link Width: x16
Link Speed: 5.0 GT/s
The 1Tb Silicon Power Nvme on slot 4 is reading and writing at about 1400/1400 speeds. The Nvme is rated 3500/3500 speeds but is bottlenecked without a Pcie switch adapter card. Pcie link speed on the Nvme is 5.0Gts.
My system is Apple native. Everything works from Apple.
49" Sceptre (UHD)/ 5k hack @ 60hz. Rx580 on Hdmi2.0 with Mojave.
Not bad for a tv screen. Wonder if the 580 will drive another screen ?
I also have the ability for monitor rotation under display preferences.
The software rescales the resolution to 1920x1080 HiDPI (5k).
Also: My tv has the ability to upscale automatically. Its a chip in the tv and has firmware that calculates frames and stuff.
There are 3 different Saphire Pulse 8gb models to choose from. The batch numbers (serial) have some slight differences, Clock speeds.
I have the 1st release serial number RX 580 Saphire Pulse 8gb.
I also disabled trim via terminal on the Nvme ssd. This speeds the system up as the system does not have to execute trim commands anylonger. (Reading/Writing)
I allow garbage collection to do all system garbage work. Ive been doing this since El Capitan on ssd drives and i can notice the difference in performance in the system. Ive never had a drive fail either. There is noticeable lag when a drive reaches its capacity however. The trick is to keep your boot drive empty as possible.
System info with this setup recognizes everything properly in Overview.
Hope this helps,