Before I changed to an Rx 580 I was able to overclock my monitor with nvidia control panel fine to 75hz on amd it wont allow me to do it in the radeon settings as amd tells me it is not compatible with my display but it was working fine with nvidia?
Have you tried adjusting your monitor refresh rate under screen resolution advanced settings? I know I had to adjust mine from 60hz to 75hz. Just follow the path after right clicking on your background- screen resolution- advanced settings-monitor. Good luck!