Shawn, isn't varying altitude with weight another justification for designing in variable Nr (over a limited range)? FADEC already has an altitude (static pressure) input, and weight could be seen as a lowpass filtered input on torque demanded. As long as Nr selected allows enough pitch margin for manouvres.
My take on fuel flow is that the engine is designed with a maximum temperature in mind for the turbine entry stage. At higher altitudes the cooler inlet air allows a higher temperature differential between combustion chamber and ambient, hence a higher pressure ratio, hence a higher thermodynamic efficiency. I imagine that turbine blades are operating at similar AOA - does N1 change with altitude for a given torque setting?
A brief bit of googling on the subject, although too detailed really:
http://en.wikipedia.org/wiki/Brayton_cycle
http://www.grc.nasa.gov/WWW/K-12/airplane/brayton.html
http://ocw.mit.edu/ans7870/16/16.uni.../chapter_5.htm
Some fun stuff if you are a sad engineer, who doesn't fly as often as you would like:
http://www.faculty.virginia.edu/riba...hermodynamics/
http://thermo.sdsu.edu/testcenter/te...r08/index.html