Puget Systems have published an enlightening test result on the comparisons of PCIe speeds:

Our testing has pretty clearly shown that for gaming using either PCI-E 2.0 or PCI-E 3.0 will give you nearly identical performance. Oddly, in some benchmarks PCI-E 2.0 was actually faster than PCI-E 3.0. At the same time, x16 was not consistantly faster than x8. Again, x8 was actually faster than x16 in many cases. So unless you care about getting up to 1.5 FPS better performance, you might actually want to manually set your video cards to operate at x8 speeds - although we really would not recommend doing so.

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