That's a nice setup, very clean and functional. What CPU, video card etc are you running? You may not be dumping enough heat into the system to really tax it. If I had to guess, I would say you're running near the peak efficiency of your waterblock, which puts you in the flat part of the curve in this graph:
Your pump can push over 2.9 GPM with no head, so even if we figure you have enough resistance to drop your flow rate to 1.5 GPM, for instance, you still wouldn't see much of a difference (less than .5 degree). Thus I don't think the pump is the problem, and the rad probably isn't either, given your results with fan speed changes.
That means that given the amount of heat your hardware is producing, the bottleneck in your system is probably the MCW6002, which is a pretty good bottleneck to have, given its performance numbers.