sonckerr,
What is (are) your purpose(s) for these efficiency estimates? Your original posting suggests an expectation of some relatively constant value for efficiency. If so, you will likely be greatly disappointed by reality.
There is nothing constant about the efficiencies of any combustion turbine system. In fact their efficiency is highly dependent upon loading. Rather than having a tendency toward somewhat constant efficiency over some significant loading range such as with naturally aspirated diesel engines, combustion turbines tend toward operating with relatively constant fuel consumption over fairly large loading range. In general, a simple cycle combustion turbine can be expected to consume fuel at approximately 80% to 90% of its full load rate (gpm or cuft/min) while operating at synchronous idle (NO LOAD).
Obviously, the "efficiency" of any engine with such a characteristic is very highly load dependent. Beyond this, the service life of critical components is highly dependent upon operating temperatures (very highly load dependent), and usually the advertised, most wonderful efficiency is produced at loadings that may significantly compromise the service life of the combustion turbine.
Highest possible efficiencies will be achieved at a loadings that will allow only a relatively few hours of operation before major maintenance and repair work will be necessary. Similarly, operation at suitably reduced loadings may permit operation for thousands or even tens of thousands of hours before any significant maintenance and repairs would become necessary.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
