when "they" say that 1080 will be as fast as 2x 980 in sli they mean with the VR extra stuff, AND those are normal 980 not 980ti etc. AND thats probably exaggerated.
In the leaked 3dmark scores my 980ti 6GB about ties and in some cases beats the 1080, either way it's close. My 980ti is a beast with a huge factory overclock but still, the fact that it's close to me says this:
Unless you have an amazing buyer and can sell your 980s for a great price (like close to break even)
OR
You're finding yourself getting even close to subpar framerates in whatever you're doing
OR
you've got tons of money and just want to be able to say you've got the newest stuff
There is no point to put ~$1200 into a new platform which will not be leaps and bounds better than what you've already got.
I guess the new games will be another factor but it will be quite some time until your system is pushed as it is and by then these things will be cheaper or you can skip a generation which is what I'm going to do. (unless something weird happens and games just start becoming too hard to run but I don't see that happening)