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.
The synthetic 3DMark benchmark is a good starting point when gauging GPU performance and Gigabyte's Radeon is showing promise: the numbers suggest that the R9 290X WindForce OC is able to offer stock GTX 780 Ti levels of performance for £480.
Not bad going, but if it's WindForce cooling you're after, Gigabyte's own GeForce GTX 780 Ti WindForce OC offers more oomph if you're willing to fork out £525.