Fan problems would at best cause 10-15 degrees as even the worst fans still move air. (having them run too slow is far worse than them being low pressure) Settings on the D5 don't seem to make much difference to temperatures either. In any case fans, flow and air trapped in a radiator would affect the temperatures of both components. Air trapped in the cpu block would be likely to cause problems. Back when i ran limited rad space, I only had problems with temperatures when stressing both components, if the cpu can't handle prime whilst the gpu is idle, something is definitely wrong.
What load temperatures do you get on the gpu whilst gaming vs the cpu? (you said 65 on the cpu whilst playing world of tanks)