The problem is, the Source engine already exhibits a lot of really bizarre performance behavior. They say that on Linux with OpenGL they're seeing performnace improvements over Windows with DirectX (with no mention of IQ), but on Windows the OpenGL engine is slightly slower (~10%), and on OS X the OpenGL engine is about 65% of the speed of the DirectX implementation on Windows and has noticeably lower image quality. Source also has wildly different performance on otherwise comparable AMD and nVidia cards. I've even had systems that used to run TF2 just fine a year or so ago, and now are a stuttery mess with the same settings on the same maps with the same number of players, for no readily apparent reason. And of course we're dealing with framerates in the 300+ FPS range for the Source engine on high-end hardware these days, where huge differences can be the result of otherwise tiny factors, as actual GPU performance is marginalized next to things like driver overhead; that wouldn't be the case for, say, Unigine or UE4.
I think if we're honest, Valve's big complaint about Windows 8 has nothing to do with "performance" or expected sales, it's more about "waah we were about to launch an application store but now thanks to Microsoft's we won't have a virtual monopoly on that for Windows."