Typically a PCIE 2.0 4x slot will have about a 5% reduction in FPS from a 16x slot.
Having personally ran a 7770 CF setup @ 16X/4X, 8X/8X and 16X/8X, there was little to no difference in performance, maybe 5% on average with about 10% being the worst between the fastest and slowest.
Toms did a test on 16X/8X, 8X/8X and 16X/4X. Note that they tested with 6950's. They are much faster and will take more of a hit than a pair of 7770's.http://www.tomshardware.com/reviews/pci-express-scaling-p67-chipset-gaming-performance,2887-10.html