It looks like you're over-volting and not "under volting". What score did you get? I'd guess around 38000, but I can't see your e-core clocks and I'm just taking a stab at how much it had to throttle when you threw 393W at it with an AIO.
 
I'm guessing you tried to adjust your V/F points, but I'm pretty sure there's a bug in the BIOS currently that prevents that from working correctly. Please share your settings and we can compare a bit.
 
Just to share some of my info, I haven't had time to tweak this more since doing some test runs a couple weeks ago, but it seems like if I put a -30mV offset on the 5400 and 5700 steps, leaving all others a 0, it will do the -30mV at my 5.5GHz all-core and limit the package to about 370W (this is with e-cores at 4.6GHz and atom core voltage at auto), giving me about 41k in Cinebench r23. If I just leave everything at auto, it'll hit that ~390W range and throttle in Cinebench pretty hard.
 
If I put a negative offset on the high end of my V/F points, the 5400 and 5700 get ignored and I'll crash because it'll apply the highest point setting to all points and I won't have enough voltage for when fewer cores hit 6GHz (my current OC). If I put negative offset for all-core frequencies and then a positive in the highest spot, it won't ever apply the negative offset for all-core. Honestly, I'm not even 100% sure that my current settings aren't applying a -30mV offset at all frequencies even though I have a 0 offset for the higher points. It seems to be better, but this BIOS doesn't even tell you what voltages started at for the frequencies, so you have to try and watch the all-core voltage and the max voltage and just do some guessing about what it did through lots of trial and error. The only setting that seems to actually work 100% correctly is the override, but then you have to try and find a happy medium voltage that doesn't cook during all-core, but is still stable with whatever peak clock you put in for fewer cores. I'd really like to get to a little bit lower all-core voltage if I can, so I definitely have some more testing to do. I was running r23 with about 360W and it was much cooler on the package, but the new BIOS updated the Intel microcode (which bumped up voltages a bunch) and I'm trying to see if I can just dial it in again without rolling back the microcode.

Yeah, 1.8-2.0 would be nuts. Also, that is indeed adding 25mV to stock, not undervolting. You'd have to put a negative voltage in there to undervolt it. Just changing the setting there will do a global undervolt (at all frequencies). It won't lock the voltage (like the override setting does), but it will apply a +25mV offset to the whole V/F curve.