italia458, here is a chart from a normally aspirated engine, which is rated at 400 horsepower, that you can play "what ifs" with.
You'll note that at your nominated 23"X2400RPM the sea level output is roughly 262.5/263 horsepower, depending on how thick your pencil is. This calculates out to be 65.7% power. You will note also that 23" is only available up to about 6,800 feet, as manifold pressure drops 1" per 1,000 feet. With a standard temperature lapse rate 23"X2,400RPM at 6,800 feet will produce 295 horsepower. The 30 horsepower increase is due to reduced back pressure, as previously mentioned, but also the throttle is now wide open and the butterfly is not throttling the induction system ie pumping losses have been reduced to an absolute minimum.
Having only 22" available at 8,000 feet you would need to increase RPM to obtain your 65%. I haven't crunched the numbers, I'll leave that to you.
