I highly recommend not running a 980 Ti using only PCIe power with one 6-pin to mini 6-pin and one 8-pin to mini 6-pin. Maybe it's ok for graphical or gaming uses which don't push the card to card 100% for more than a few seconds at a time, but not for applications that use the card near 100% for sustained periods, e.g., machine learning. I killed the backplane of my Mac Pro 4,1 flashed to 5,1 like this.