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!
I was apparently up way too long when I read the original post, lol. Yes adding the power to the riser takes the load away from the motherboard slots if you only connect the card's riser wires to the PSU. The power draw I still believe is equal from all ports, but I am not 100% sure.
It is best to have power connections from the same PSU for 1 GPU (in other words, don't have 2 power cables from 2 different PSUs connected to the same card) to avoid weird issues.