Nvidia GTX Titan X Benchmarks
#21
The GPU will hang very quickly if the frequency is unstable.

You'd have to do some serious overvolting to get much more of an overclock out of these GPUs.
#22
In my personal experience, Furmark/OCCT/FP64 LL tests are way more stressing than hashcracking.
#23
Those don't stress the ALUs though, which are all we care about.