Well, I got bored and tried a few things. Note this is a full watercooled system 7820x on an Asus X299 Deluxe w/ 360x60mm dual radiators and a 1080ti FE at stock clocks whole testing in the mix.
For the below I set XMP Profile on (G.Skil TridentZ RBG DDR4-3200-C16 sticks at 1.35v).
AVX is set at -5, AVX512 at -8 <--- questionable need.
At 4.5 -> Adaptive, set to - at -0.100 / Turbo 1.150 / Input Voltage 1.6 = stability showing 1.144v
At 4.8 -> Adaptive set to - at -0.020/ Turbo 1.150 / Input Voltage 1.7 = stability showing 1.233v
Failure on 4.5 at -0.110 and 4.8 at anything under -0.020
Given the rather large jump needed to get from 4.5 to 4.8 I think I'm just going to settle on 4.5 and call it a day. I played Wildlands, The Division, WoW, Rise of the Tomb Raider and Shadow of Mordor for a few hours and had no issues; everything ran much smoother. I didn't have HWiNFO running for most of it but the separate thermistor display in my loop was showing 45-48 celcius the entire time.
I'm going to try some further undervolting on the input voltage. That seems to drop temperatures quite a bit at idle but I'm trying to localize where exactly. The little generic OLED display on the motherboard (this is a funky thing) showed the drop and the 'CPU voltage' at 1.608v when I set it to that 4.5 stable result.
If this is the wrong place for this report let me know; there seems to be no other real non-dick swinging thread to put it in.