Regarding filling empty PCIe slots I think these server power supplies does not have PCIe power connectors? So we are limited to cards with power from PCIe slot ony or using external power supplies? I have a Supermicro quad socket Opteron 6276 machine but CPU mining only on that one. I figured out that by using external power supply and powered risers it may be possible using GPUs too but right now I have a separate GPU mining rig.
Breakout boards... All my HP server iron have GPUs running along with the CPUs. Boxes like the r815 that don't have the onboard connectors to power them but could easily run GPUs with the addition of external PSU and risers.