Update on the 300w HDPlex with the 1080ti...
I was able to use it to play Mass Effect Andromeda for about 20 hours and it held up right until the end, but near the end of the game it would crash as soon as the game loaded, unless I underclocked the card far below stock. Using normal clocks, the system would shut down. What was also very apparent is that the coil whine tone of the HDPlex had changed. So I think running right up around 300w for so long caused something on the PSU to actually degrade where the PSU's wattage capacity is permanently diminished. I didn't have this issue at all with the GTX 1080, so I'd recommend against pairing the HDPlex 300w with the 1080ti, seems like it's a bit too much. I did have a fan hitting on the HDPlex and the case is open, so it was being cooled decently.
Edit:
I think I have a new idea that I might try. Maybe I can use an HDPlex 160w PSU and dedicate that to the CPU/motherboard, and wire it in a dual-PSU configuration to the HDPlex 300w, so the 300w is dedicated to the GPU only. I'm
http://www.hd-plex.com/HDPLEX-160W-DC-ATX-Power-Supply-16V-24V-Wide-Range-Voltage-Input.html