The air is thinner when flying high, so you need to run the prop at a higher angle of attack to get an adequate "bite" of air. For this reason, i run RPMs a bit lower at high altitudes than what i usually run at low altitudes. This also gets rid of the "shaking" from the engine running rough.
I discovered this while flying a QMB mission starting at 4km of altitude. I could fight with ease all the way up to 5km and during the cruise back to base i even took it up to 6km without any problems at all.
As for emergency power, if you are high and the outside air pressure is low enough it comes to a point where even the supercharger can't provide a sufficiently compressed volume of air to the engine because the starting point (the outside air) is at too low a pressure.
Finally, leaving throttle on full and regulating only the prop pitch can work, as long as you stay within a certain power band of RPMs and a certain band of airspeeds. You can do way cooler stuff if you work the throttle and prop together though. What you do is like driving a car with manual transmission and regulating your speed by shifting gears without ever taking your foot off the gas pedal: it limits your effectiveness.
If you work both controls at the same time you can force enemies to overshoot easier, use the prop as an airbrake (idle throttle and fine pitch) or pick up speed faster in dives and so on. Hope this helps getting you started, cheers
