Just for clarity, it'* an M62 and not an M60. Eaton'* naming convention for their superchargers is based on the displacement of the rotors per revolution. In this case the two compare as follows;
M62 = 62 cubic inches/revolution or 1 litre/revolution
M90 = 90 cubic inches/revolution or 1.5 litres/revolution
You can find more details on Eaton'* web site;
It'* not necessarily an indication of efficiency when looking at boost in PSI terms. If you have a small supercharger pulley on a car with a restrictive exhaust you will get boost stack. This occurs because the volumetric efficiency of the engine is not sufficient to injest the output of the supercharger and therefore the pressure builds.
The opposite of the above occurs when headers are installed. Since the flow will be improved, therefore achieving better volumetric efficiency, then the boost will drop in pressure.
Hope this helps.