Depends on the game, Fallout 4 doesn't see that great an improvement on average (indoors there is a huge improvement) but outdoors the game still has those weird framerate drops where it can dive from 90 to 50 fps in certain open spaces for no apparent reason. Averaging around 60fps.
Shadow of Mordor sees a huge improvement, going from high on my 970 to mostly ultra on the 980 and also gaining around a 40-50 fps increase! Playing at a steady 100fps.
Just Cause 3 increased the settings from medium to high and also see a 20-30fps improvement. Averaging around 75fps
Assassin's Creed Syndicate: Again gone from medium to high but also seeing about a 20fps improvement. Averaging around 80fps.
So overall, very impressive. It's definitely required on a 1440p monitor if you want to maintain at least 60fps without compromising on the visual settings too much.