The 6-pin will allow 75W as known, even though other users differ when the R9 295X2 came out lets just say it's a given for now. 75+75 (two 6-pin power connectors) + 75 (motherboard power) = 225W, The GTX 1080 has a TDP of 180W. It'll work fine. If you wanted to use molex to 8-pin power connectors that is also fine, that will allow for maximum power draw if it ever needs it. This is assuming you know your power supply has enough amps on the 12V rail to power the GPU sufficiently.