There is a small difference in performance between full x16/x16 and x8/x8 SLI. The actual difference varies between games.
The ability to use SLI and the performance gains far outweigh the few percentage points difference between x16/x8 modes IMO.
Keep an eye out on the HardOCP site. They're getting ready to publish an article next week that shows what kind of real world gaming impact running x16/x8 SLI will have using SLI GTX 460s