PCIe 2.0 x16 = 8,000 MB/s = 64,000,000,000 bits
PCIe 3.0 x16 = 16,000 MB/s = 128,000,000,000 bits
GTX 980 Memory Clock = 7,000 Mb/s = 7,000,000,000 bits
As you can see, PCIe 2.0 x16 can EASILY handle the mere 7 billion bits with its 64 billion bit threshold.
Source:
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-980/specifications
http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.0