IMO, PCI-e 3.0 will be stalled for a while.
In my experience, I've tested some PCI-e 2.0 and 2.1 GPUs on both PCI-e 1.x and 2.x, ran some tests, went back and forth and TBH, I can't say there is no difference, but I'm totally sure, no one can tell by experiencing but by numbers derived from...