This graph neatly sums up what this week's conundrum has been at work. That is the shaft horsepower vs the altitude at various steady state speeds for a Rolls-Royce R Type 11. The problem with all the equations for thrust and thrust specific fuel consumption is that at the root your turbocharger or your turbojet/turbofan/turboprop all depend on temperature and pressure.
Someone's compiled it all nicely to show the affects at high altitude. Specific thrust depends on both temperature and pressure, and when both of these drop so does your horsepower. Or more importantly, since the main goal of any jet engine or turbocharger (like the turbocharger in your car) is to make use of high air flow for extra power, this can be a problem when at high altitudes that air flow is effectively reduced. As the picture shows, you can make up some of that by increasing your speed and artificially increasing the air flow through your engine, but not by a whole lot.