Valve claimed in their internal tests it did not meaningfully change performance, which I'd be inclined to believe. The Steam Machine is likely going to be most often GPU bottle-necked, so the CPU performance regressing by even double digit percentages doesn't necessarily result in any change in gaming performance.

You'll note in that techspot comparison, by contrast, they used the fastest CPU and fastest GPU and then still used medium/low settings to really maximize whatever difference the RAM speed would have. Which is a valid test, but it's not necessarily going to generalize to low-end hardware. Like the CPU being limited to 90fps instead of 120fps doesn't matter when the GPU is struggling to hit 60fps in the first place.