Hey max,
Your CPU only supports 16 PCIe lanes, so, if you ran the GTX 980 on the x16 slot, and added any other PCIe add-in card (sound, RAID, USB, storage etc), it will force card into a slower speed.
Also, if you decided to buy a second GTX and run them in SLI, they would each run at x8 (x8x8) -as opposed to x16x16 on newer mobos/CPUs
This can be a bottleneck..BUT -as long as you are just running that card and nothing else in any of the other PCIe slots- the card will function at 100% performance -assuming its not a dud card!
Hope this helps!