it will work fine the only reason theres powered raisers out there is for cards that dont have a psu connection (motherboard powered) anything with a cable that goes to the PSU should work perfectly fine
There's an alternate situation where powered risers are needed, and that comes about when you power the entire rig off a 12VDC supply, using a small step-down converter that just powers the motherboard + CPU. In that scenario, it's better to supply all 12VDC power straight off the supply rather than feed all the extra juice through the step-down converter and motherboard. In some tests, our 5850's and 6950's do draw very near 60W on the 12VDC lines from the PCIe slot in addition to what's coming in on the separate PCIe power connectors on the GPU's.