...and this is what I get for posting with less than 2 hours of sleep.
Essentially, the GTX 1080 is connected like this
https://imgur.com/a/OJC94bQ#wgrNW4V
I have no idea why that molex-looking Y piece is there, pulling an 8pin connector out of two 6 pin ones.
Theoretically, I should get rid of it, install the 2080 and then use one 6 pin and somehow put the other 6+2 pin one together, to form an 8 pin and just stick those in, minus the Y piece, correct?