3DMark Fire Strike
3DMark is the latest version of this hugely-popular synthetic benchmark. Making use of DX11 features such as tessellation, compute shaders and multi-threading, it provides modern-day results and is available as a free download.
3DMark Fire Strike remains a reasonable proxy of actual gaming performance. Any card that can score over 10,000 marks at 1080p and 5,000 marks at 1440p should be able to play most games at those resolutions with a combination of solid frame rates and decent image quality.
As it is, the Sapphire RX 580 card is about five per cent faster than the also-overclocked RX 480 OC. Every little helps.