TC, I think I am correct in saying the Gazelle Astazou engine is slightly different as it is a fixed spool engine compared to the free power turbine of most other helicopters. Like the fixed wing turbofan the fixed spool engines have oodles of surplus performance at sea level and the transmission torque is the limiting factor - the N1 is constant and so therefore is the mass flow through the engine, the only variable is the amount of fuel squirted into the combustion chamber.
As you go higher with the Astazou it becomes more efficient as more of the mass flow is used in combustion until eventually all of it is utilised. Since the only way to get more power out of the engine is to throw in more fuel the T4 becomes the limiting factor on this type of engine unlike a free power turbine where the N1 limit is usually the stopper.
The design is good for hot and high which is why Allouettes (also fixed spool) are still used widely in mountain ops.