My take on your situation is that you can sell that 980 ti or use it in another build. The GTX 1080 is monstrous and on par with a two-way 980 sli ( And even surpasses it ). The 1080 duo can run The rise of the tomb raider at 5K at 60FPS !!! It can also run other titles like BF4 at 100fps. ( high - ultra settings ). A dedicated PhysX card cannot improve your performance by more than 10 frames or so, but it won't hinder your performance.