Gary Lager: surely the efficiency of an engine is not solely determined by its thermodynamic efficiency? As I said, it's a while since I looked into this, but a quick Google threw up the following links:
A Wikibook (I know, I know!) about
jet propulsion containing the following phrase: "A typical high bypass engine will consume about 8mg/Ns at maximum takeoff and 15mg/Ns at maximum cruise thrust."
A
report about the GE90 where SFC is given as 15.600 mg/N-s in the cruise and 7.910 mg/N-s at TO. The engine would therefore appear to be almost twice as efficient at sea level compared to cruise altitude.
Clearly temperature is an important factor, but I don't think it offsets the density loss at altitude. Mach number would also play a role I think.