@severniae Do you have sources to this claim about efficiency? Either for the TBM specifically or for constant speed props generally? I'd like to understand this more.
To quote Daher from the cruise performance pages in their TBM 850 POH:
Propeller RPM utilization between 1600 and 2000 RPM is possible without changing
performance. Display the TRQ indicated in table with Np = 2000 RPM, then reduce Np
without exceeding 121.4 % TRQ.
In other words, any theoretical reduction in propeller efficiency is not worth considering when reducing RPM, at least not in this aircraft, as long as you're not butting against a limitation like in your example. And above ~FL250 you're no longer able to reach 121.4% at 2000 RPM, so reducing RPM should increase TQ for (within reason) the same performance, not the loss of TAS that we currently see.
What is yet to see is if this is something BS can remedy without going completely outside the sim like Majestic did for the Q400, or if the age old issue with Microsoft sims' poor rendition of turboprop engines will make this a permanent thing (and if FS24 will be any better than it's forebears)