It's like AGP 8x, it was somewhat of a wank. Even 4x didn't benefit greatly over 2x.
PCIe bus speed becomes critical if there has to be constant access to main memory for textures and stuff. In the case where the card has a nice big amount of RAM, it can hold a buttload of textures anyway.
There's also cases where there might be blits from main memory to the card for CPU generated graphics e.g. emulators or some desktop games but again the bus speed won't be super critical.
A PCIe 2.0 x16 card has bandwidth of about 8 Gig/sec vs about double that for PCIe 3.0 x16. In most cases either is plenty.Go to the full post