Here is the single card comparisons:
http://www.anandtech.com/bench/product/1496?vs=1493
As you can see, even if SLI scaling was around 190-200% (fairly unrealistic) the 980Ti would be on par on slightly below. Not to mention the power, heat, SLI idiosyncrasies, and noise that would be generated using two 780s over one 980Ti.
If this question pertains to "Should I upgrade or not?" I would say if you would pay for a 980Ti out of pocket, no. If you sell the 780s towards the purchase, yes. Especially when you consider that the longer you wait, the more the value of the 780s will drop.