Ryu - I wouldn't be eliminating the PCIe slot power from the GPU, just from the mobo.  The DIY powered riser tutorial has you take the power wires from the riser and add a 6 pin PCIe power connector to it which means your other supplemental power connectors don't have to pick up the extra load, you would now have 3 cables from the PSU to the GPU, assuming you already had 2!