1920x1080 = 2,073,600 pixels
2560x1080 = 2,764,800 pixels
That's 33% more to render.
If you can get 90+ FPS @ 1920x1080 that will 'roughly' translate to 60+ FPS @ 2560x1080. (* Performance is not really linear, but this is 'close enough' for an estimate.) So you want to check your current performance at 1080p to tell if you need more GPU power or not.
You should be OK with a 770. But no harm in checking first.