Do you guys even read Anandtech?
🙂
Scroll down to "PCIe 3.0 Bandwidth Comparison":
http://www.anandtech.com/show/5458/the-radeon-hd-7970-reprise-pcie-bandwidth-overclocking-and-msaa
Note that, more or less, a PCIe 3.0 x4 slot has the same bandwidth as a PCIe 2.0 x8 slot and a PCIe 1.0 x16 slot. There are measurable but not super huge drops in performance on the 7970 at the PCIe 3.0 x4 level vs PCIe 3.0 x16 - 8% on Shogun, 18% on the Dirt 3 minimum frame rate. PCIe 2.0 x16 and PCIe 3.0 x16 are much closer, implying that PCIe 1.0 would be a minor restriction but 2.0 vs 3.0 wouldn't matter at all.
And Meaker10: the charts clearly show that different games are influenced differently by lack of bandwidth. Some don't care at all and others see noticeable drops when artificially limiting to x2 (see Batman vs Shogun on the second chart.)