It's not that hard to calculate the pumping loss. Expanding a volume of 720 in^3 = 1.18 x 10^-2 m^3 against a pressure of 10^5 Pa costs 1.18 kJ. At 2400 RPM, that's 80 pumps per second or about 100 kW or 130 hp. So if you reduce the ambient pressure from 1000 hPa to 800 hPa, you get 20% of that, or about 26 hp back as useful work. Close enough?