As a matter of interest, I am busy working on a Allison / RR turboprop conversion onto a M20R (one off for now). I have the turbine and aircraft and busy doing the engineering and got a provisional green light from the authorities for an initial 12 months (with an possibility of extending) of testing once it is ready to fly. I also have lined up a “Category 1 test pilot” friend who would do the relevant flight testing(in the past he has done flight testing for both EASA and FAA) I am throwing this out there as I was wondering what sort of interest there would be in a future STC. I am very familiar with the Allison/RR 250 series engines as I also fly helicopters that are equipped with them so fuel burns etc at lower altitudes (unpressurised flight levels) were part of engine type selection.