No, imacs are designed to be unupgradable. You can update the RAM in newer models and the HD easily in older models ( you can also upgrade HD in newer models but it's a lot of work and could easily mess up).
We'll see a more fair comparison with 980ti vs 1080ti later this year but I was hoping the leap would be more pronounced as the 980ti is struggling with some of the current resolutions/VR applications.
Sorry to disappoint @Dexter benchmarks at 7:50
@willfly4bacon Its 2 way native, nVidia clearly said that you can do 3 and 4 way with the beta drivers