Please read my posts before you answer.
You can change the voltage on any card which has the capability to be changed, which requires more expensive components and not all cards have it, so you would be best to ask on the Sapphire forums if your specific SKU is able to, as there are several different SKUs of any model.
I have ASUS GPU and here someone had changed voltage in EXACTLY THE SAME model as mine http://www.techpowerup.com/forums/showthread.php?t=125836
As for the CPU, you need to use BIOS for overclocking, especially if you are using a core unlocker feature. You also do NOT need to be using Cool N Quiet with an unlocked CPU, as the disabled core(s) might not be able to function at the different C state voltages.
I use unlocked CPU with C'n'Q for more than year and everything is okay.
If I set multiplier in BIOS it can't be changed in idle. When I use software, multiplier and voltage decreases.