3DMark
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.
PowerColor's R9 390X Devil is right on par with a stock-clocked, reference-cooled GTX 980 in the synthetic 3DMark benchmarks. What's interesting is that the card isn't a great deal quicker than Sapphire's R9 390X Tri-X, which ran at close to reference speeds. Seems the extra core and memory overclock isn't going to give PowerColor a major advantage.