2x 980 are better than a 295x2 the same way a single 980 is better than a single 290x.
The 295x2 is roughly the equivalent of a pair of 970's so you could expect 15-20% better performance from the 980's and they should use about 100 Watts less power as well at peak load.
It's hard for a single GPU to run 4K at 60 fps, even with an R9-390X. It will take a couple of years for that to happen. Perhaps the next-gen "Pascal" or "Volta" NVIDIA GTX TITAN III (or whatever it will be called) will have a chance.
2x 980 are better than a 295x2 the same way a single 980 is better than a single 290x.
The 295x2 is roughly the equivalent of a pair of 970's so you could expect 15-20% better performance from the 980's and they should use about 100 Watts less power as well at peak load.