Ok, slow down and pull the reigns back!
I stumbled across this thread and went and looked and found a new BIOS for my Z170A-X1/3.1 and updated it. Upon reboot I see the option in the lower right as POST is happening "Press X to enable X Boost SKY OC"
. I did and now I can successfully OC my i5-6400, and on a cheap mobo to boot! This option was never there before and was orginally only possible on a special BIOS and then was squashed like a bug by intel. But now it seems ASRock has silently released this again without any fanfare??
I have successfully bumped up to 4.0 right now and tested, and it definitely seems I will be able to go more, but I got to get my cooling in order first because now my CPU temps are not reported correctly, (which was the way it always worked, so that tells me IT IS working, woohoo!). To answer your question about the mV I am on +200 right now with a BCLCK of 150. (Please keep in mind I am no expert overclocker and I have a 6400 not a 6500). This video helped guide me since its ASRock specific, but there are still some things a little different, so you will have to experiment, as with all overclocking.
I googled this "new" feature and shockingly hardly found anything about it. Did I miss the train that ASRock did this or is just that new...? Also keep in mind there are other things that get disabled like iGPU so you will need a discrete GPU, and some CPU instruction sets. Imo its still best to splurge on a K if you want to OC but see what you can with what you got for now.