Instead of a 1070 OC, get a 1080 standard card. Will run a lot cooler, be more stable and you can OC it as an option. Some games do not handle OC GPU's very well and if your card is OC out of the box, you don't leave yourself wiggle room in case of issues.If the 1070 OC cards match its performance you will see the 980Ti going for a lot less than that.
As for the 980Ti prices coming down, I don't see it happening soon. I paid $659.99 US for mine on Amazon and a price check shows the current price is $659.95 which is a four cent difference from February. If prices were falling, Amazon would be leading the pack. Demand for HMD's will go up for a while and the cards that can drive them will be in demand. 980Ti is a damned good card and it's not going to lose a significant amount on the asking price any time soon.