I think the 120 "not enough power" issue relates more to the gearbox limitations that the engine. Hence I can't see value in producing (say) 800 SHP from the test motor and simply trashing the gearbox.
Whilst the current approach is great for testing the motor, surely a production aircraft would have a new gearbox, doing away with the idea of a multiplier box. In that case it is a perfect time to increase the blade area and uprate overall power.
Not for "cheap retrofit" (obviously) but a very sensible solution and still minimising the additional certification / testing.