Epsylon check your PMs dude.
With memory under-clocked I get 281MHs, even with only 1 card. At mem -500 Nvidia auto adjust my core clock to ~2090mhz which is pretty high. There is no way I could get 300+?
Is cuda 8 bad? I noticed the screenshot above the guy is using cuda 7.5.
Or maybe Nvidia 367.27 drivers are slower? I use them because overclocking in linux didn't work with some recent ones I tried.
Newer isn't always better. Sometimes it is, but often times it's not. Kernels need to be retuned to new versions of Cuda and most developers don't do that. Even when they take the time to do it, it doesn't always end up faster.