Assuming you're undervolting (and not increasing power limit,) you can get away w/ using only one of the pcie 6+2 connectors.  A single connector can carry 150W (plus another 75 from the board.)  Just brought 2 Sapphire 580s online (one regular w/ Samsung, one SE w/ Micron,) having never plugged in the second connector.  And yes, they're stable - at least for the past 24 hrs

Btw, do you happen to have any max performance numbers for a Sapphire Nitro+ 580 8Gb w/ Micron (SE preferably.)  Getting ~900-925H/s w/ 2230@0.925 after just using the PBE one-click (which I believe is just a strap copy-up for Micron,) and I'm wondering if I should bother trying something custom (like your 5xx strap from above.)