All this information is out there, just google it.
Okay, okay, I'll google it for you:
Average improvement on 2560x1440: +30%. On 1920x1080, that number is +20%. On 2560x1080, it'll be pretty much bang in the middle so +25%. Not a lot, but noticeable. Personally, I would want +50% improvement, although with other improvements like lower power, better cooling, better noise, more VRAM or newer features, I'd be ready lower that requirement. Comparing the GTX 680 to the R9 290, you get
more power consumption, but double the VRAM, not really any new features to mention, and cooling & noise is not really relevant here if you watercool it. Only +25% is quite difficult to justify in general, but since you struck a trade deal, that's cool.
It's going to vary by game, see the
full review.