I see no reason to why an 6Gb/s SSD would not work with your motherboard.
x8x8 makes no real difference, a few percent loss in worst case. I doubt your graphics cards will exceed the bandwidth, especially when that motherboard has PCIe 3.0, which has twice the bandwidth of the previous generation.
Besides, most motherboards don't even support anything higher than x8x8 when only duo SLI.
Just look at the most extreme model from Asus, the Maximus VI Extreme, which is barely two months old.
It boasts a single at native x16, dual at native x8/x8, triple at x8/x16/x8 and a quad at x8/x16/x8/x8.
Anyways, x8x8 or x16x8 or whatever doesn't really matter, and if it does, it's only marginally. Perhaps in future years, if Moore's law holds, it will be bottlenecked, but not at the current moment, no. By then the bandwidth will probably have been expanded anyways.