mentalray for Maya 2015 SP2 is now available and provides important fixes around XGen hair rendering, layering shaders, massive assembly scenes, native IBL and more. Here’s a short list of fixes that come with the new mental ray 3.12.1.16 and the new version of the mental ray plugin.
Please, consult the mental ray and the Maya 2015 SP1 and SP2 release notes for the complete list.

• XGen
  The performance of the XGen hair shader has been significantly improved for scenes with dense hair.
• Layering shaders (Mila)
  Several fixes in the shaders and the UI – namely, possible crash with subsurface scattering initialization, possible NaN values and a too bright contribution of the user IBL environment shader in glossy components, reordering layers with bump or weight connection.
• Binary Maya file compatibility with Maya 2014 restored
  Some mental ray shader nodes saved in .mb files were not compatible between 2014 and 2015. This is now fixed.
• GI GPU
  Improved performance of the GPU and multi-CPU rendering, and added support for NVIDIA Maxwell GPUs.
• Large scene with assemblies
  Fixed possible crash when rendering large scenes with multiple assemblies exceeding physical memory.
• Native environment lighting
  IPR now automatically updates when changing the new “emit light” features which control the native environment lighting. Fixed brightness difference in IPR versus Render Current Frame.

Happy rendering,

Julia