CyberCrist, I was able to get 6 EVGA 750Ti FTW cards running on the ASRock Z87 Killer.

BIOS
PCI-E 1 (gen2)
PCI-E 2 (gen1)
PCI-E 3 (gen1)
Also disabled integrated graphics and changed PEG0 De-emphasis Control to -6dB

PHYSICAL
PCI-E 1 (x1)   --> 750 TI (1x riser, shorted)
PCI-E 2 (x16) --> 750 TI (1x riser, shorted)
PCI-E 3 (x1)   --> 750 TI (1x riser, shorted)
PCI-E 4 (x1)   --> 750 TI (1x riser, shorted)
PCI-E 5 (x16) --> 750 TI (1x USB riser, shorted)
PCI-E 6 (x1)   --> 750 TI (1x riser, shorted)
PCI-E 7 (x16) --> EMPTY

It's messy at the moment and I have to clean up now that it's working but here are pics for proof.  It's funny, we're very close in the equipment we used  I tried using a bootable Linux USB stick but had issues so I'm using an old hard drive.  I have an EVGA 750W PSU but it's only pulling about 460W from the wall.  These cards run very cool.  I don't have a 7th card to try and I likely won't.  For some reason I couldn't get a ribbon riser (also shorted) to work in the PCI-E 5 (x16) slot and when I tried a USB riser, it worked.  I have a monitor plugged into the PCI-E 2 (x16) slot.  Just like you I don't want to mess with it now that it works.

I am upset that I can't overclock these cards in Linux.  There's so much more potential.  What speeds are you getting with all 6?  I am getting about 1550 KH/s mining Litecoin with CudaMiner.