Not much to add apart from the link http://www.techpowerup.com/reviews/NVIDIA/GTX_980_PCI-Express_Scaling/ Interesting to see that 8x is sometimes faster than 16x on PCI-E 3.0.
1 card isnt going to be bottlecked in any way on 8x 3.0. 8x 2.0 is only a slight bottleneck. If you run 3 or 4 cards at very high resolution the bottleck will be much more severe.
+1 4 Titans @4K sometimes show a significant drop in performance switching from PCI-E 3.0 to 2.0. The most dramatic example I have seen is using 4 x 290Xs @4K again switching from PCI-E 3.0 to 2.0 with a drop in performnce of about 25% in some games. What was interesting was if only 2 cards were used in both of the above setups then there was practically no loss of performance switching from PCI-E 3.0 to 2.0