Win 8.1 x64, i7 5820K, 16GB ram
3X GTX 980 Ti
Broken: 2.74, 2.75RC1
Worked: (not yet)
Short description of error
I recently upgraded my primary workstation from 3x GTX 980 graphics cards to 3x GTX 980 Ti cards - but the performance of the new cards is actually about 1/3rd of the performance of the original 3x 980s. What's strange though - these cards score very well on all CUDA and OpenCL benchmarks I can find, but in Blender, these 3 cards working together actually sometimes underperform my single GTX 680. What's strangest is that these cards do show a tiny performance boost on the Blender 2.7 BMW benchmark scene over the previous hardware configuration, but in every scene which I design, they're performing much, much slower. There does seem to be a small correlation between which shaders are used in the scene, as some shaders seem to be effected worse that others. Ambient Occlusion for instance, is hardly effected at all relative to the 980s.
I have tested the new cards with the Blender 2.74, 2.75 RC1, and a few nightly builds since then, and this issue seems to effect all current versions.
Exact steps for others to reproduce the error
Using an NVidia GTX 980 Ti, render the following scene to see dramatically hindered performance relative to the hardware potential of the card.