While I'm fine-tuning my miner a couple of conclusions so far:

Hardware:
MSI Gaming 5 Z97
7x XFX Radeon 480 GTR Black
7x USB Risers (blue) v.005S, Windows 10
PSU: FSP Aurum Pro 1200W 80Plus GOLD
8GB DDR3 1600 low voltage
CPU Intel G3220T @2.60GHz, no mods

Conclusions:

• PEG Gen setting in BIOS - doesn't seem to impact stability, affects power drain slightly (average from 10 measurements taken each 10 sec after achieving a stable temp):
  - Auto/Auto/Auto - 928W
  - Gen2/Gen2/Gen2 - 927W
  - Gen1/Gen1/Gen1 - 919W - stayed with this one
  
• PCI Latency - seems to impact stability
  - Latency 128:  time to failure: 5min42s(BSoD), 21min31s(BSoD), 1min26s(GPU0 fail), 12min03s(BSoD)
  - Latency 32: 2 hours and keeps on running stable - stayed with this one
  
• Windows 7 - don't use it for 5+ GPUs, use Windows 10:)
  
• Other settings: onboard LAN adapter, audio and video disabled, PCIe set to 8x/4x/4x of course
  

Voltages and frequencies are of course main factors of performance/stability and now I'm playing around with best performance vs. power drain
At the moment I'm mining Zcash using Claymore's miner v. 9.1, -cvddc 975 -cclock 1225, intensity 8 and seems stable, Average total 1523H/s, power drain (from the wall) 903W, which gives 1.69H/s/W. Fans running quite at ~30%, GPU temps ~70C, ambient temp 24C, PSU fan also quiet. GPU's BIOS is slightly modded (mem timings to improve ETH mining, quiet fans and lowered voltages at top states)