I can confirm 6 1080 Ti with 2 850W PSUs have no problem.
Viable if you run them at about 230 watts or less (180 ballpark for the power supply that is ALSO running the MB/RAM/HD stuff) - and your power limit setting is RELIABLE (which is not always the case on Windows, Afterburner among other options will sometimes LOSE settings on one or more cards after a driver reset).
I wouldn't push them much if any harder than 700 watts at the wall even on a GOOD power supply, for long term reliability reasons - even with the TDP turned down, you still get SPIKES on power draw quite a bit above the set point.
On an overall "system cost" basis if you are aiming for max efficiency, the 1070 ti beats the 1080 ti on a hash/$ basis AND on a hash/watt basis - but the 1080 ti system will have a bit more ability to expand hashrate *IF YOU HAVE ENOUGH POWER SUPPLY CAPACITY* and are willing to live with lower efficiency - and the figures come out pretty close, you're not going to see a 1070 ti based rig making money and a 1080 ti rig losing money unless you're pushing the 1080 ti cards a LOT harder.
At this point, on both the 1070 ti and the 1080 ti my "go to" card is the EVGA SC version.
I like my Gigabyte Aorus 1080 ti cards, but the dual-8 pin power connector makes them a headache to get multiple cards hooked up compared to the 8+6 on the EVGA SC, and since I'm aiming for good efficiency the cooling on both cards is plenty and they perform equally 'till you push them up past about 220 watts (the better cooling on the Aorus lets it boost higher at that point for a bit higher hashrate and efficiency).
The SC also tends to cost less than the Aorus, but on a $750 ballpark card the $10-$20 usual difference isn't a big deal in and of itself.
5 x 1070 ti also fits into the power budget of my rigs, where I end up having to run only 3 x 1080 ti and set them to a somewhat less than best efficiency point (and STILL end up with less total hashrate).
Down side is the 5 card rig WILL take up a fair bit more space - but even there it's not an issue in my specific case as my current "shelf/rack" design fits a 5 card rig very nicely on a single shelf with good cooling, but would be overkill for 3 and WON'T fit a pair of 3-card rigs on one shelf without major cooling issues.