Mike Irish sent me a link to this KitGuruTech video today. If you go to about the 6:30 minute mark, you'll see him test the same bit of video with three video cards, starting with GTX980, and then a GTX1080Ti and finally a Vega 64 Sapphire. If you ignore the fist result in the table below, which is from a different system, the results are remarkably close, and based on this, you'd have to think carefully before spending serious money upgrading a 900 series card to a 1000 series card. It might be that his testing regime was flawed, and I don't know how you'd be able to tell that from the information in the video, but it is food for thought on the face of it.
I am sure Bill has done similar tests, and will have his own set of results, and I am also sure that Bill's findings would hold more credibility.