No difference. Again: no difference in performance!
Gameplay performance can be summed up very easily and directly. We experienced no differences in the gameplay experience between x16/x16 SLI or CFX compared to x16/x8 SLI or CFX. There were measurable differences, differences that some people might pick apart and declare a clear "winner," but we don't see it this way at all. For gamers, the differences are so minuscule that the delta simply does not matter. The fact is you have to run at insanely high graphics settings just to detect the one to three frames per second differences we documented. With lesser resolutions and quality settings the difference in frame rates becomes so small that it is not detectable. Then in some instances we actually saw the x16/x8 configuration run faster. We don't have any big explanations for you on that, but the results were repeatable.
We found that at 2560x1600 we managed to see the most differences because we were running with 4X AA enabled at the highest in-game settings in AvP and Metro 2033. These games are notoriously resource hungry in terms of texture and shader performance. Pushing these titles up to 4X AntiAliasing has an impact on overall performance, memory capacity, and bandwidths. But in terms of PCIe bandwidth, we still only saw a few frames per second difference at best. When we raised the resolution to 5760x1200 we experienced even less of a difference because we had to back settings down in order to play the game. This points out that there are other bottlenecks that we run into before we even think about the PCI-Express bandwidth being an issue. Not that it truly was to begin with.
The more we learn about PCIe Gen. 2.0 the more we have to love about it. While PCIe Gen. 3.0 should be upon us in the coming year, we have found that we have a lot of usable PCIe 2.0 bandwidth now, at least as far as we are concerned with multi-GPU video card configurations.
You can find slight framerate differences between x16/x16 and x16/x8 SLI and CrossFireX video card configurations if you go looking for them while pushing games to super high end resolutions and quality settings, but nothing that negatively impacts a real world gaming experience. If you are wanting to use SLI or CrossFireX on that motherboard that does not support x16/x16 PCIe lanes, have no fear. If you do have a x16/x16 motherboard and want to space those hot video cards out a bit to let those breath a bit easier, well have at it.