[...] this game engine does not model performance variation from sea level to high altitude.
It does, but you have to select "Show true airspeed" instead of "Show indicated airspeed" in the realism options. I can guarantee that every supercharged propeller aircraft will see the performance increasing from sea level up to the supercharger critical altitude after which it begins to drop off. Indicated airspeed is good for realism but useless for performance testing unless you know exactly how much altitude and speed affect the airspeed indicator reading - and that's something you probably don't know. Neither do the rest of us. I can't speak for others but I've never used IAS because I want to know how fast I'm going, not how fast the pitot tube thinks it's going in air layers of varying density.
Having said that you'll want to change the airspeed option anyway before you begin testing because you'll get nowhere trying to make the aircraft perform like they should at 10.000 feet if you have IAS on, if you happen to get it right (it certainly won't be right although it may look like so) by making the IAS reading to match what TAS should be at that altitude the sea level speed will be completely out of whack as your aircraft is flying a lot faster at 10.000 feet than the airspeed indicator says.
The game engine is actually very good what comes to climb rates and level speeds - if you have an aircraft dialled in at a certain power level so that its climb rate and maximum speed match those of real life, you can change the engine output the same amount as in real life and get the same performance changes as in real life - but it has one serious shortcoming, it treats all engines as single speed single stage supercharged versions. While that works for aircraft like the Bf 109G/K series which had a large single supercharger driven through a hydraulic coupling with the rpm changing depending on altitude, or turbocharged ones like the P-38 and P-47, multi-stage superchargers never go through the phase of losing boost before the higher gear kicks in. They just give a steady high boost level below the critical altitude.
Well, that was completely OT when it comes to the speeds at 10.000 feet but the bottom line is,
change that airspeed option or you'll be doing a lot of useless work. After you've changed it check the maximum speeds again, they should be noticably closer to reality. And if they're still way off (or you have true airspeed selected already) the flight models in question aren't worth fixing because doing it right would require replacing half of the data fields...
