Assuming Nvidia's VR SLI pans out, dual 970 TIs should get you 80% of the perf of dual 980 TIs for a cost comparable to a single 980TI. But, wait until VR SLI is demonstrated and tested in the wild. Until then, the added latency of AFR SLI is counter-productive to VR.
I don't think anyone can predict the future as there is currently so much optimisation going on across all the involved parties - Intel, Nvidia, Microsoft, Oculus, Valve, Epic, Unity, etc. They are all focussed on getting the best performance and experience out there.
This is a great summary of what Nvidia are currently doing:
https://developer.nvidia.com/sites/default/files/akamai/game...