[Cathar]

Uh, that's what the Hydraulic Power vs C/W graph tries to show you. Shows performance relative to a fixed pumping power, which at a glance provides a decent summary of the performance across a range of blocks with a fixed pump, or at least pumps of similar characteristics.

A pressure vs C/W graph favors lower pressure drop blocks.
A flow vs C/W graph favors higher pressure drop blocks.

A power vs C/W graph treads the middle ground. If you want an exact value you still need to plot the particular pump's pressure-flow curve against the system's pressure-flow curve and find a flow rate, and then look at a flow vs C/W graph to derive a performance figure. The power-C/W figure tries to present a generalised summary of that sort of information though. It is a generalisation, but it's a lot closer to hitting the mark for users who just want to look at a single graph and nothing else, to presenting an even handed appraisal of the efficiencies of various waterblock designs when matched with any given pump.