Benchmark Results: Just Cause 2
The GeForce GTX 570 loses 2% and 8% when placed in x8 and x4 slots, respectively, compared to its original PCIe x16 benchmark scores. The Radeon HD 6950 doubles those losses in the same transitions.
Even at 2560x1600, a x4 slot costs the Nvidia and AMD cards 6% and 12% of their original performance, respectively.