No that GPU is bottleneck at PCIE 2.0 due to 2600k processor on 4k and it is a reference card.
PCIE-2.0 should not be limit here and the CPU seems to max at 75% in that GTX 1080 video so not really be a bottleneck. The FPS shown there (55 - 62FPS ish) seems pretty good for a GTX 1080 (see GTX 1080 review FPS with a i7-6700K here : http://www.techspot.com/review/1174-nvidia-geforce-gtx-1080/page4.html)