AMD issued a statement regarding the performance variability of its Radeon R9 290 series graphics cards:
Based on feedback from the enthusiast community, we’ve implemented an all new PowerTune mechanism in the AMD Radeon R9 290 series that exploits the full capability of the individual GPUs rather than clamping performance to a least-common-denominator type of capability level. This has the advantage of improving overall performance but does result in some degree of performance variability. These changes will also result in some degree of run-to-run test variability based on environmental and operational conditions in un-controlled test environments.
The range of performance differential is not expected to meaningfully change the user experience but we’ve taken note of recent reports that the degree of variability is higher than expected. Reasonably we would expect the variability to occur both above and below the performance of the press samples, however it appears that most reported performances are biased towards the low side. We are actively investigating these reports and we will update when we have completed our investigation.
It is important to note that is it to be expected that the more thermally limited the setting the more variation can naturally occur. AMD Radeon R9 290X, by default, ships with two default BIOS settings for different acoustic levels and the "Uber" setting was designed to limit the level of thermal throttling and comparisons in this test mode are more consistent. Furthermore, AMD have implemented comprehensive PowerTune controls within the OverDrive panel of Catalyst Control Center and users can tweak to their own desired balance between acoustics and performance.