Phil you're awesome!
I have some of those motherboards on the way, also I have a z170 with 4 PICEx16 - spacing is going to be tight for a 4 card 1080TI no riser but I'm going to run it just to see how long it can go. Should get here monday.
My new bottleneck - these stupid Corsair 750's only have 2... TWO! 4+4/6+1 power ports. The Rosewill 850w you suggested does have 4 PCIE power lines, so I just need to get some more cables that have 2 sets of 6+2's in them, I ran out. I upgraded a dozen 1070's to 1080Ti's today - hardware always takes longer than you expect. Was expecting to unscrew, unplug, swap and be done. The power cords threw me for a huge loop.
I was thinking about running power from my 1500w EVGA but I actually don't have long enough cables for that either.
So back to the drawing board.
https://www.newegg.com/Product/Product.aspx?Item=N82E16817438061This EVGA GQ 850W has EIGHT 6+2pin. You could technically put 4x 1080 Ti on that Biostar mobo.
Just make sure to get the blower versions of the gpu and lower the power limit to around 75 to 80.