You get 3x better performance with a GTX 780 Ti than a GTX 980 Ti. From reading the explanation from a Nouveau developer in the comments, I get the impression that NVIDIA just wants the open source drivers to work well enough that you can boot your computer and download the proprietary drivers.
I think the biggest missing piece currently is signed PMU firmware for GM20x to allow reclocking, which NVIDIA has not released and does not seem to currently be planning to (even though they have released some other GM20x firmware).
You get 3x better performance with a GTX 780 Ti than a GTX 980 Ti. From reading the explanation from a Nouveau developer in the comments, I get the impression that NVIDIA just wants the open source drivers to work well enough that you can boot your computer and download the proprietary drivers.