ConclusionAfter testing both AMD's Radeon HD 7970 and NVIDIA's GeForce GTX 680 on Windows 8 using 19 benchmarks, we can confirm that both Windows 7 and Windows 8 offer the same gaming performance. Some games at low, CPU-limited resolutions show Windows 8 lagging slightly behind Windows 7, which is a sign that there is some mechanism at work here that increases CPU time or otherwise limits the system from sending information to the graphics cards fast enough to achieve higher framerates. This difference is very small though and barely noticeable, especially at higher resolutions like full HD or 2560x1600. The same performance is, at higher resolutions, pretty much the same as in Windows 7. The difference we noticed would be smaller, or go away, with a weaker graphics card at lower resolution as the slower GPU will limit framerates in such a scenario.
In terms of driver quality, I have to commend NVIDIA for releasing a perfect-working, rock stable Windows 8 driver, even though it is still marked as beta. AMD's first Windows 8 WHQL certified driver doesn't do so well here. We spotted a significant performance loss in Crysis 2, limiting maximum FPS to 63 - no matter the resolution. Other games work fine though; this seems to be a limited issue.
Overall, there is no reason to upgrade to Windows 8 just for a performance gain in games. There are certainly other reasons that make the OS upgrade interesting like faster boot time, UEFI support and Storage Spaces. There are, on the other hand, some turnoffs like the Metro UI that many users describe as clunky.
Support for DirectX 11.1 is, at this time, useless because there are no games that support it, nor have games with support been announced.