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It's not actually that much, if you can get your hands on an Eaton TVS model.
The geometry of the housing and rotors is designed to take advantage of acoustic effects, and the rotor sealing is not great so generally you see high efficiency at high speed.
A centrifugal supercharger loses efficiency at lower than design pressure ratio, and a turbocharger loses efficiency because they rely partially on backpressure to generate power. A Roots supercharger equipped engine maybe loses 5-10% more power than a turbo pushing similar boost, but who cares because you're not always on boost.
If you actually want to calculate it, you can approximate it (since you don't know how the intercooler will affect temperature just yet). Compressor flow is 1x rotor displacement per revolution * volumetric efficiency (which you can look up on the chart), pressure ratio you assume adiabatic compression, then use PV=nRT or the formula for work in adiabatic compression to estimate the temperature increase, which you can derive work per unit mass airflow from assuming ideal efficiency. Then divide by the efficiency written on the compressor map and you'll get your number.
On the supercharged Mustangs and Corvettes, they say it's around 40hp for 9psi boost (for a 6L engine at 6000rpm ish). If you only want 120hp from a Honda Insight, I imagine it can't be more than 10hp even with an older blower.
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