Efficient means in this state that it is taking LESS power and LESS boost to make the same amount of power as an older...LESS efficient blower would...example...GenIII. Comparing all the outlets, inlets, blower displacement, rotor coating materials, and initial size of the rotors changes everything.
This might be a good comparison.
The 1st generation M62 supercharged 3800 Series I used a 2.9" pulley from the factory...made over 10psi...and only made like 200 or so horsepower, because it was horribly inefficient.
The 3rd generation M90 supercharged 3800 Series II used a pulley NEARLY an inch larger in diameter (stock 3.8" pulley)and made 40 more horse...on nearly half the boost...worlds better than the previous Gen M62...but still a Heaton blower.
The 5th generation M90 supercharged 3800 Series III used the same size pulley...and they were rated 20 horse more BUT...more prone to knock because of the fact that then GenV flows much better and is way less parasitic due to better intake tracking, rotor coating/materials as well.