Almost 9 months between the 980 and 980Ti.
I don't think SLI is going to be that useful in the near future, support seems to be waning. Maybe in a few years when all the games you want to play are DX12 and Asynch capable, but by then a new GPU will be available.
980 GM204
980Ti GM200
1080 GP104
Titan X GP102
GP100 is available as a compute only GPU, while there are Quadros based on the fully functional GP102
A 1080Ti is a tricky aspect. The Titan X (Pascal) is a slightly crippled GPU with 12GB of memory. With the previous Titan X having double the memory of the 980Ti, they didn't really leave the room for that here. Doesn't look like HBM2 is on the table for GP102, and GP100 has stayed away from gaming GPUs, and is already the same core count as the Titan X.
You might recall the GTX780/Titan/780Ti which had performance in that order. The 780Ti was the fully functional GPU. They could do that here and release either a Titan X Black or a 1080Ti that has all 3840 potential cores enabled. That is what I am hoping for at least.
But I don't see it happening unless demand for the P6000 is low. If they are having yield issues making 1080s then this larger chip is going to be that much harder. Might be why the Titan X popped up so quickly, they can make crippled GPUs all day.