^ actually the performance on the 2560x1080 is much closer to 1080p than 1440p. Simple pixel count can tell you that.
the performance impact will be ~25% compared to 1080p, and ~40% better than 1440p.
Anyway,
the GTX 960 (the GPU is the component that affected by changing resolution) is not that powerful card and you will probably upgrade it soon. monitors are for many years. so you can get an UW monitor and if performance is too bad, you can lower a bit some graphical settings (like shadows quality) and the performance will be back.
The problem with UW monitors is not the performance impact, but the games compatibility. Not all games support 21:9 out of the box, some do not support at all and some do support in a very annoying way(overwatch is the most annoying).
So before getting UW monitor, check that your favorite games actually support this resolution.
For tasks other than gaming, the performance impact is next to none, but the experience is so much better