I thought I would post this comparison since there's very little info about the RX 590 in a classic MP 5,1. If you are considering the Sapphire Pulse RX 580 for the cMP 5,1, you might want to look into the RX 590 as a newer and faster Polaris card. As of this writing the 590 is $30 more on Amazon. I tested the MSI RX 590 ARMOR and the Gigabyte RX 590 Gaming cards. I kept the Gigabyte RX 590, and I will tell you why. The RX 590 replaced a Vega FE air-cooled in my cMP 5,1. Vega FE's raw performance was good, but I was not satisfied with its inefficient thermal to performance ratio and the unbearable fan noise. Some have said under 10.14.5, the Vega FE behaves better, but it's still a hog and I don't need that much bacon for a 5,1. I wanted a card that's recently fresh and can keep a good pace with the next 2-3 iterations of MacOS until the 5,1 is no loner supported. I sold the FE for enough to buy two RX 590 cards. I settled on testing the MSI RX 590 and Gigabyte RX 590 cards to find the best suited for my use. My criteria were: 1. A more recent card than the highly recommended Sapphire Pulse RX 580, a 2 year-old card outperformed by the RX 590 by about 12%. 2. Reasonably power efficient, around TDP 200W. 3. 2-card slot wide. Of the 2-slot RX 590 cards, the MSI and Gigabyte were the most affordable and sold directly by Amazon. I didn't try the ASrock RX 590, another 2-slot card, which was sold by an Amazon seller and could be harder to return. 4. Could be backward compatible with FCP legacy under Sierra. This eliminates the Vega 56/64 cards. RX 590 Performance benchmarks -- In synthetic benchmarks and real-world tests, they performed about the same. The Gigabyte has a slight edge on my machine. FCPX BruceX test: Gigabyte - 16.5 seconds. MSI - 17.5 seconds. Unigine Valley: Gigabyte - 1920x1080... Gigabyte - Preset Extreme HD... LuxMark v3.1 -- left Gigabyte, right MSI: Unigine Heaven 4.0: MacOS 10.14.5 Beta does not yet properly identify the RX 590 cards in System Info and About This Mac. They are listed as Radeon Polaris 8 GB. Both cards work OTB in Mojave 10.14.5 Beta. EDIT: 10.14.5 and later properly recognizes the RX 590 as "Radeon RX 590." Both cards fit into PCIe slot 1. The MSI is bulkier, 1 inch longer and .75 inch deeper. The Gigabyte is more compact but wider by several millimeters due to the external backplate heatsink. There is only about 3 mm of clearance between the Gigabyte and my Amfeltec Squid card in slot 2. The Gigabyte RGB logo cycles through different colors, and the "fan stop" LED indicates the fan status at 0 RPM. The MSI has no RGB. The MSI requires two 8-pin power cables. The Gigabyte requires one 8-pin. I kept the Gigabyte because it seems to handle power consumption and thermals better than the MSI. The Gigabyte semi-passive fans idle at 0 RPM. The MSI fans idle at 600 RPM yet still very quiet. In the BruceX tests, the Gigabyte fan speed was 843 RPM vs the MSI at 1090 RPM.