I happen to have the same monitor (albeit the Freesync version - XR341CK) running on 2 x AMD R9 280x. You must remember that you have gone from a pixel count of 2073600 to 4953600...so over double your original count. I would expect a halving of frame rates on that basis alone.
Your choices are to lower graphics settings (bloom. lighting) and certainly turn off Anti Aliasing. Beyond that your looking at upgrading that 980 to a GTX 1070 (or a GTX 1080 if you're feeling like a generous dad!!!!)