Overclockers Club published a review on the XFX Radeon RX 580 8GB GTS Black Edition
A quote from the article:
What surprised me most about this card when I was overclocking was that I was not able to get it above a 1440MHz clock speed no matter what I tried. This is 88MHz less than I was able to push out of the PowercColor card I just looked at last week. Kind of a downer after those lofty results, but not every GPU or CPU is going to deliver stunning OC results. That does not mean another card will overclock better or worse. It just means this card was not the best overclocker. Temperatures were not a limiting factor while overclocking this card, with a maximum temperature of 57XFX Radeon RX 580 8GB GTS Black Edition Review @ OCC