Sorry, probably I'm not quite correctly state my opinion:
To obtain the maximum hashrate  all GPU's must be installed in the PCIE x16 slots that run at the maximum possible speed (ideally x16).
But most of the motherboards I know allow x16 speed only in one slot, if you install GPU's into the remaining PCIE slots,then their speed is usually less.
From the specification of this particular EPOX motherboard: Graphics slot   8 x PCI-E 16x (1PCI-E3.0x16 + 7 PCI-E3.0x1).