Considering that PCIe 2.0 effectively doubled the available bandwidth over 1.x, I would think that a PCIe 2.0 SLI/CF setup @ 8x would still be faster than PCIe 1.1 @ 16x. But in all reality, you really wont see the difference between the two as no cards can saturate the available bandwidth.
I saw a review where they put a 5970 and a GTX 295 in x8 slots, and they each lost about 20%.
As far as what it takes to saturate an x8 slot, a 5850 is where it will start to hurt performance, with it becoming more noticeable with higher clocks. Anything more powerful loses more.