Seasons Greetings Old Smokey,
My reference to Mcrit was that Mach No will have to be considered.
If flying higher at LRC Mach Number results in an IAS further away from best IAS (as Bookworm showed) - then there will be a penalty.
I didn't know that max engine efficiency is "typically in the vicinity of 90% N". I thought engine efficiency improved with higher temperature and pressure, so up to MAX Cruise power was more efficient.
I will do some homework.
Regards, RRR