one factor is engine type. a mechanical has no real way to know how much lower the pressure is, and delivers fuel mostly on engine RPM and governor input setting with perhaps a little boost pressure monitoring to limit full fuel application until adequate boost is detected.
so less air = more fuel or richer mixture at no/low boost. but since the governor is delivering fuel based on RPM, it will taylor fuel delivery to whatever rate is required to maintain its pedal commanded RPM. Probably the most felt at startup as the engine is cranking and the rack is at the startup position without any boost and delivering fuel for what is normally atmospheric pressure. but again as soon as it fires the governor will adjust fuel to meet required idle RPM.
The mechanical wastegate actuator typically pushes against a mechanical spring, and that doesn't change with altitude, so within its limits to do so, under load the turbo will add more air to reach that same wastegate spring pressure and should continue to mostly deliver rated boost pressure... Starting with a thinner source air, the turbo will probably eventually hit a point where it is unable to continue to make rated boost as altitude increases. Depending how severe this gets it may cause the governor Fuel Ratio Control(anti coal device
to limit full fuel rack/delivery... but once you run out of boost you are probably going to over-fuel if the engine cannot reach the commanded RPM.
Now the EFI component of HEUI systems probably looks at atmospheric pressure(I think the C7 does, the 3126 may not?) which could help it taylor delivery based on atmospheric pressure. This might only really be noticed during startup and un-boosted conditions, but again, since we are not throttling air and fuel and an established A/F ratio to control the engine RPM, the system will deliver the fuel it deems necessary to reach the RPM commanded by the RPM/speed selector(right pedal)...
O course neither system monitors final result like a gas system does with O2 sensors and closed loop regulation...
There is a term in aviation called turbo normalized, usually a rating with an altitude associated with it, like 10,000FT, which means that the aircraft will have the same HP at that altitude as it does at sea level.
In terms of a diesel, it should also apply. your turbo normalized altitude will be the altitude where you can still achieve full boost pressure form the turbo. when peak boost starts to fall off, you have used up all your turbo's overhead and the maximum torque you can create will start to fall off and your governor/ECU will have a tendency to over-fuel if you cannot meet commanded RPM...
