I just wanted to say that both working at x8 isn't necessarily bad.
You don't get twice the bandwidth in and out, but you DO get twice the processing power when you are in.
I think that bandwidth in and out is less likely to be the bottleneck than the processing power on the inside is.
That being said, everyone's situation is different and I don't know yours.
It is hard to notice a difference between x16/x16 and x8/x8 unless you are using 3 monitors on pretty high graphics settings with high end video cards, according to the reviews that I have seen. The x8/x8 even comes out ahead a lot of times because everything the video cards do doesn't need to be routed through a NF200 chip (the video card equivalent of the scenic route).
+1 Couldnt have said it better.