The theory on the Nvidia forums is that the latest drivers either purposely crippled Kepler or did nothing for it to get everyone to move to Maxwell or some shit, but Nvidia are denying it and said they're investigating the issue. All Kepler GPUs specifically have seriously shitty performance for some unknown reason. 290x destroying a 780 Ti (it's always the opposite), a 960 is beating a 780 (780 is usually 36% ahead of a 960). Titan (original) beating the 960 (BY ONLY 2 FPS). They estimated a 40% performance penalty for Kepler for absolutely no known reason.
It's clear the Kepler GPUs have been fucked over in some way, which is the claim other sites are making as well (It doesn't take a genius to notice, just look at the benchmarks, they make absolutely no sense...), and there's also a long reddit post (it's apparently accurate) claiming that the GameWorks libraries are what crippled Kepler. I don't know what's going on man...
But yeah... Nvidia has commented saying they're investigating the issue... As if they'd do anything.
All of us with Kepler are basically fucked thanks to Nvidia's shitty business practices.
Edit: People on Kepler are getting performance improvements rolling back driver updates. Look at the Nvidia forum.