The graphics cards won't be drawing from the PCIe Riser as long as you have the PCI-E connected directly to the video card.  My rigs run just fine on SATA power connectors directly to the Riser.
6 GTX 1070 + MB + CPU + RAM + SSD = 75 watts idle and 860 watts at full hashing load (cards set to 90% power).  I run that 860 load on a single 1000 watt PS