K80 is 2-slot and has higher aggregate DP performance than Titan Black.
K80 is ~2.9TF (peak aggregate DP throughput at maximum boost)
Titan Black is ~1.9TF (peak DP throughput at maximum boost)
note that peak DP throughput at maximum boost is generally not sustainable (on either card). So these numbers are just for relative comparison. K80, like titan-Z, achieves its “aggregate” performance via the contribution of 2 GPUs. K80 is also only available in a variant that is intended to be used in a qualified server platform. It is relatively less suitable to “build-your-own” solutions or use in an arbitrary workstation or PC.
I don’t believe there is a single CUDA GPU solution that outdoes Titan Black on DP, at this time.
Titan-Z of course does outperform Titan Black in aggregate, and I suspect it may outperform K80 in some cases as well, but it is a 3-slot solution (and dual GPU).