The engines are not doing any more work at FL400 than at 10,000 for the same IAS. 
The IAS at 10k results in a TAS not much greater than IAS.  At FL400 the IAS TAS difference is much greater.  At FL300 the GARMIN GS was 514 kts at .84M while the IAS was 340 kts.  The difference GS to TAS is wind.  Assume for illustration no wind.  While atmospheric pressure is much less at altitude than at 10k, the OAT is far less also.  Standard temp at 10k 23.3 degrees F; at FL400 it is -67 degrees.  The jet operates on fuel added to increase temp for a thrust reaction.  Low temperature slows the speed of sound and at FL400 can result in drag due to transonic shock waves.  Thats why the wings are swept and great attention given to faring of the wing, tail, and fuselage.  For real, the 747 would not carry a full fuel load for the Orlando/Frankfurt run.  Costs $ to transport fuel not required for flight plus a reserve.  Cargo  produces revenue.