You will not have any problems in the near future (at least 2 years). PCI-e 3.0 doubles the bandwidth of PCI-e 2.0/2.1.
PCI-e 3.0 x8 = PCI-e 2.0 x16
So what depends on the bandwidth saturation? The hardware or the software? For example, when more complex games come out in say 3 years and my gtx 680 will work harder and have better drivers by then, do you still think that the 3.0 x8 lane going to be enough?Is it only when i'll get a new gpu that I may have a problem?