iGPU Overclocking on i7-5775C

So I thought I'd try boosting the speed of the iGPU (Iris Pro 6200) slightly on this machine as they typically overclock a little for a decent boost (it's a little integrated thing hence no discreet card).

Anyway, the system in question uses an Asrock Z97E-ITX/ac motherboard that allows all sort of control over the iGPU including frequency and voltage. I've tried raising the iGPU frequency from 1150 to 1250, but it doesn't seem to have any effect. Tried again to 1400, doesn't appear to have done anything at all. It still seems to be running at a max of 1150Mhz.

The machine is running Linux, but I'm not sure that has any relevance on the issue.

Any ideas why?