But my Asus R9 290's have their own power connectors, 2 x 6pin PCIe connector per card. Shouldn't powered riser be useless in this case?
Yeah I read that the first time, The answer is still no.
If you do not have a power connector on the motherboard specifically for the pci-e slots you need to use powered risers, because the motherboard traces and the wire feeding them aren't thick/wide enough to carry the power for all of those cards
What is the brand/model of your motherboard?