For goodness shake gentlemen. You have been in the business long enough to know that head pressure alone is not the factor.
So is MP39 run higher head pressure. Now a day A/C use 410A which runs at 389.6 PSIG (at 115 degrees saturate temp) is a bad thing?????
It is all in the pressure and temperature relationship and the total amount of heat eject/absorb.
MP39 will phase out in short future and 409A is not. Tha's the only advantage.