I truth I feel the GPU's today have such good coolers on them already that they really do not need water cooling. Unless you are going for looks more that performance I would skip water cooling the GPU but that just me. I was going to water cool my Sapphire HD 6950>6970 at one point but ultimately skipped it.
As for the RayStorm I have found it to be a very good block. I have upgraded my system with that block and the RX240 rad for my AMD FX-8120 and FX-8350 rigs.
If you go for cooling the GPU as well make completely sure that a 680 block will fit correctly on your 670. You said it is a non-reference PCB do you mean it is a 680 reference PCB?