Such a gpu solution is not worth it imo. Better to get a single gtx 1070 at that price. More stable (single gpu setup), less power consumption and heat. I guess the performance of Xfire rx 570's is around (a slight advantage for the RX 480) Xfire Rx 480's since an RX 570 is within 5% of an RX 480. I would wait for AMD Vega (due to launch in a few weeks) and then decide.
Single RX 570 vs RX 480: https://www.youtube.com/watch?v=8i7IrDRyt7M
P.S. I am considering clock/clock performance not stock settings.
You can just google it but for the sake of this forum here
http://www.reviewstudio.net/2475-msi-rx-570-gaming-x-4g-and-crossfire-review-great-2k-gaming/performance-%E2%80%93-4k
3D Mark is a good indicator but does not tell you actual gaming performance and not all games support multi-gpu configs although right now support is pretty good.