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Turbocharged engines are NOT immune to altitude, that much I know for sure.
I don't know of any CAR manufacturers that tune their turbos like AIRPLANE manufacturers do. In the turbo airplanes that I've flown, the general idea is that you can take an engine that makes 235hp at sea level N/A, and make that 235hp up to something like 14,000' and then slowly drop off in power at even higher altitudes, but generally not as much as if it was N/A.
This means the turbo is essentially doing nothing at sea level (just making up for slight losses in tubing and induction bends, leaks, spinning turbines, etc, at something only marginally above ambient pressure).
These engines are designed to operate continuously high altitude and at full power with the turbo running to produce it's "max rated horsepower", that is what it's designed for. So at high altitude, the turbo is spinning and so on. It's also real cold up there by the way.
A car's turbo is usually designed to be working hardest during maximum engine output, at sea level. If this weren't the case, you'd see turbos having a much shorter lifespan in places like Denver and others as they'd be working harder, but this is not the case. My turbo made maximum pressure at sea level (guage) and as I got higher and higher, it bled off gradually. I live at high altitude, so I saw the effects every day. The other issue is that it takes longer to "build up" the boost at higher altitude, so even if you can tune it for higher pressure, it lags more in getting there, but as far as I know, no car manufacturer actually designs and tunes their engines like aircraft manufacturers do. If anything, the turbo engine doesn't "drop off" as rapidly as the N/A engine, but the turbo's efficiency max is reached rather quickly if you are just going up in altitude and it may be spinning super fast, but not making all that much pressure anymore.
In any case, a simple way to think of it is turbo aircraft engines use turbos to maintain displacement as altitude increases. Car manufactuers use it to increase displacement at SL. Lot of people think that in a car it works like an airplane. It doesn't.
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