"Our test bed is a Dell XPS desktop, running a 3.2GHz Core i7 processor, with 8 GB of DDR3 RAM, a Nvidia GeForce 8400 GS PCI-Express with 512 MB RAM graphics card, and a 1 TB Western Digital hard drive"
With specs like these the differences aren't going to be much, but on older hardware or more "typical" of what the average user has Windows 8 will perform better. A real test of Windows 8 performance is to take a range of typical spec'd older and newer machines and run both OS's on them to compare. This is the reason Windows 8 isn't going to boost PC sales, unlike Vista users won't need to upgrade in order to run 8. I installed W7 on an old Dell Latitude D620 Laptop with a Centrino CPU and 2GB ram and performance wasn't that great, so much so that it was sitting on the shelf for years. I installed W8 on it and it runs fine and its now being used again, it even plays HD video without issue which it couldn't do before without stuttering.