Aren't they bandwidth?
How could that not make a difference, 8x and 16x, are you serious?
..because it matters how much information is being passed.
By this, I mean that when you're only using 4x, 5x, or 6x worth of bandwidth then whether you have that information traveling over an 8x or a 16x buss makes little to no difference in actual performance.
Also, please keep in mind that newer boards will use PCIe 2, which has double the per lane throughput of the earlier standard. So a 16x PCIe1 buss has the same capacity as a newer 8X PCIe2.