I just buy 4x Gigabyte Rx 560 4gb. They are not bad at all. With bios mod 13.7-14mh/s per card with 75w. Stock they are 12-12.3mh/s. The only problem is that they need more space. I think can reduce the power consuption up to 40-45w, but I will try this later. If Asrock start seling there new mobo with 13 slots this alternative can make more hashrate then 6xrx580, but new nvidia cards are on the front door and after some days it seams pointless to buy this cards, just because 14mh/s is the max and you need more slots. IF the new Gp106-100 can take up to 30mh/s will be the best gpu ever for mining.
Put the Gp106-100 GPU out of mind... it was reported from a website that has been known to be unreliable. Other rumors said it would only be sold in bulk to large miners anyway.
Either way, we live in the now and the reality is, RX 560's are a pretty good value along with GTX 1060. The density is low with RX560 but provided you can get enough of them setup for cheap, it shouldn't be a bad option.