It's not the bottleneck. I doubt there's be any difference at all in performance between PCIe 16x and PCIe 4x, unless there was a glitch in the chipset somewhere.
Hmm, I'm a little skeptical on this one. Why would they impliment a new technology like PCI-e and not use it's bandwidth. If PCI-e 4x is just as quick as PCI-e 16x and PCI-e 4x is just as quick as AGP 8x, then what's the point in PCI-e?