Thanks for your initial responses. My brain has just managed to further confuse itself. The wing doesn't know or care if it is at sea level or FL390. The IAS/CAS determines the performance of the wing. Why doesn't the same apply to the engine? If you are blasting through the less dense air at FL451 in a 747-400, for argument's sake at (MADE UP NUMBERS) Mach 0.85 and CAS of 230 Kts, isn't the engine ingesting / compressing air at the equivalent of 230 kts at sealevel? Isn't the ram effect creating the same conditions as sea level? If the wing doesn't care about TAS, only ACTUAL air flowing over it, ie IAS/CAS, why is the engine different? Why is is improving while wing remains subject to CAS?
If the same amount of air is passing through the engine, why does the fuel burn lower from 230kts at sea level?