@NKD, take your PC, choose a clock for your GPU and stick with it. Now run Furmark with your CPU @2.0, 2.5, 3.0, 3.5, 4.2 and show your results. I bet they are all within 1% of each other, probably even closer.
Next. Choose clocks for your GPU and CPU and stick with it. Now run Furmark with your GPU with memory clocks (main GPU clock same) increasing. Plot the results. Any difference? If there is, then there is a memory bottleneck. If it doesnt increase, or it is only marginal, then there is no memory bottleneck. (For furmark at least)