As it happens I have the same device and same desire to reduce the awful fan noise at load.
I read your tip about turning off Turbo Boost and lowering the voltage (thanks) and then independently ran into the same problem in having my XTU settings not restore after a reboot.
It turns out that they actually are restored but there is a delay - maybe up to a minute. I observed the core voltage drop in AIDA64 as it apparently applied my settings. Strangely by launching XTU you can accelerate the process.
I can see why the delay makes you think it's forgotten your changes, it fooled me.
By the way, how did you arrive at the -25mV drop, trial and error?