GTX 1080 have a special memory and this memory don't work well for mining activity.
Then 1080 hashrate is not better than 1070's hashrate and they eat a lot of watts more.
Sometimes, 1080's hashrate is lower than 1070's...
This. Having both 1080s and 1070s working I find the 1070s almost match the 1080. I cannot speak about special memory, but performance is darn close, and I can tweak the 1070s down to under 110 watts and get almost the same numbers. I only bought a couple 1080s so when I retire them I can run them in a game rig, but they pull more power and are barely better.
Also $459 for a 1070 is very pricey. Look for prices more like $400 to $420