Ideally if you can measure accurately the compressor inlet and discharge pressures/temperatures, this would be the criteria by which to judge the improved performance. Sadly this is seldom the case. It is better to judge the performance improvement by the fall in exhaust gas temperature (EGT) after a wash. If the EGT is measured at a specific power setting before and after a wash, a more efficient (i.e. cleaner) compressor will show a fall in the EGT. There should also be a fall in the fuel flow.