I finally decided to "OMG GO EXPERIMENT YURSELF!!", like the kind mods here suggested. I'm putting here my results, in case someone with nothing but basic knowledge about voltage configuration wants to avoid frying their CPU, if only a little (BIOS 0430):
100% Default BIOS settings: 1,25v (VID and Vcore).
Default with SVID Best case: 1,1 Vcore 1,16 VID (Referred as "Good values" from on now).
XMP ON, NO improvements (SVID Auto: Same as default BIOS settings. SVID Best: Good values).
XMP ON, improvements (SVID Auto: Same as default BIOS settings. SVID Best: Good values).
Literally any other configuration I've tested with MCE disabled: 1,344 (Bad values).
Note that in none of these configurations my CPU hit more than 4,3 on all 6 cores. So frankly IDK what the MCE option or that "CPU improvement" prompt when activating XMP does.
My final recommendation? Load BIOS defaults, activate your XMP profile, select "NO" in the prompt, put SVID at Best Behaviour and you're good to go (any further suggestions are welcome).
And pray both that those are the default stock Intel voltages indeed, and that ASUS fixes this complete mess.
Tests done with Prime95 and Cinebench, measured with HWiNFO64.
Also, I'm adding some live tests made with Destiny 2, Crysis 3, CEMU (Zelda BOTW), and Star Citizen Alpha 3.0:
Destiny 2 / SC 3.0: Voltage stays whitin the 1,1-1,16 range. VID roughly the same.
Crysis 3 / CEMU: Voltage/VID normally stays whitin 1,18-1,21 range (hopefully they're still whitin reasonable values, although I've read people having 1,2 for OC'ed CPUs).
All of them with MCE disabled: 1,344-1,36 VCore, and around 1,30-1,32 VID.
My temperatures with Bad values were 65 C, and with Good values are 50 C (both average).