Just installed the latest NVIDIA beta driver – 361.18 – and it is now OK with Plasma and KDE5..
Release notes and download link here: http://www.nvidia.com/download/driverResults.aspx/97474/en-us
Notes include: ”Fixed a bug in the EGL driver where a mutex was unlocked more than once. This triggers undefined behavior, and in particular, if lock elision is enabled in glibc, may result in a segmentation fault.”
* ”lock elision” is not a typo – it is described here: https://lwn.net/Articles/534758/
Robert Gadsdon. January 13, 2016.