http://www.hardocp.com/article/2015/06/18/msi_r9_390x_gaming_8g_video_card_review/9#.V9hp0K1SQ6U
The GTX 1060 is more on par with a GTX 980 than a 970. Since there was never an 8GB GTX 970 there are no comparisons but I'd say it would look very much like the 290X/390X test above.
Yep about 10% better than the 970, which is about where the 980 sits. The 980 was always poor value IMO. The same argument can be made for the 980.
1) The GTX 1060 has the same performance as the GTX 980.
2) The GTX 980 has 4GB VRAM and the GTX 1060 has 6GB VRAM.
3) Cards are supposed to have VRAM amounts that are sufficient for their performance levels.
4) Therefore, either the 1060 has too much VRAM, or the 980 has too little VRAM.
IMO the 1060 would have been fine being 4GB. But would that have lowered its prices? Probably not so I'm happy they went with 6GB.