Re: X299 Dark VDroop versus Z390
X299 DARK was designed earlier and didn't have this function implemented at release. Secondary reason for that - VCCIN input rail is less sensitive to voltage variations, because motherboard does not provide power directly to CPU Cores (like on little socket), but to FIVR on CPU package instead. So I take loadline on X299 VCCIN rail bit less important than it is on little sockets. And CPU on X299 to which this would matter more is now EOL (KBL-X).
I'll test additional options (VRM on both boards is nearly identical) and hopefully BIOS guru's will add this into upcoming X299 DARK BIOS versions.