I just installed a ASUS radeon RX 560 installed in my MP5,1. I get no video, but 10.13.6 recognises the card in PCI devices when GT 120 is installed as well. My screen also registers a connected device when HDMI or DVI is connected, but no signal. Initially I thought it needed external power but there is no connecter on the card and it seems it runs fine on power from PCIe Current boot rom: mb51.007d.b03, I cannot install Mojave and upgrade to newer firmware as the GT 120 does not support metal. As far as I can tell the card should be supported and run fine on High Sierra. Is it only the SAPPHIRE cards that works with the classic MPs?