There is a turbo multiplier on my Crosshair V Formula bios, and you can adjust the cpu voltage as you would normally to accomodate a higher turbo boost; however, my own experimentation and what I've read online indicate that the only effective way to increase the turbo is by using the AMD Overdrive software. As far as I can tell, no matter what the turbo multiplier is set to in the bios, the turbo is hard locked to 3.9ghz on 4 cores and 4.2ghz on 1 or 2 cores.
I've unlocked a higher turbo on 6 cores (e.g. 4.6ghz) using Overdrive but I find the software is not intuitive and the settings are not retained on reboot even with the "Apply my last settings when system reboots" is checked in the preferences. It may have something to do with that green "boost" button about which there is inconsistent info online and limited instructions in the Overdrive Users Guide.