yr right ...
You're right!
I initially didn't 'join the dots' from your post, but it's all coming back to me now.
From what I can recall, there was nearly a 50% increase in MTOM under the STC for the modification to that aircraft.
The NZ CAA addressed increases to MTOM with modifications in Civil Aeronautics Manual (CAM 8), as follows:
7 – Weight and Balance
7.0 General This section stressed the importance of weight to the structure, of weight control and balance, and that if approved limits are exceeded then flight tests should be made.
7.1 General Effects of Gross Weight Changes This section begins a discussion on the relationship between the design load factor and gross weight. It made the point that gross weights chosen should permit safe operation under all normal and emergency conditions. The chart that is currently reproduced in CAR Part 137 Appendix B is mentioned as a guide to determining gross weight provided the aircraft is flown in a restricted manner.
7.10 Effects of Gross Weight Changes on Aircraft Structure This section noted that the aircraft landing gear and supporting structure are particularly critical if the aircraft is landed overweight; and taxiing is very likely to be unfavourably affected by increased gross weights.
With the increase in gross weight there must be a proportionate decrease in the load factor that can be reached in flight. It again notes that caution should be exercised in all flights at overload weights whether or not they are below the possible maximum. There was some discussion on the distribution of load with respect to the structural strain on the fuselage
7.11 Effects of Gross Weight Changes on Maneuvers This section notes that to prevent excessive loading the aircraft must be maneuvered cautiously, and that the stalling speeds are increased and stalls in turns are more easily encountered. A lower allowable load factor means restricting bank angle. Another factor is gust loads; the level flight and never exceed speeds should be reduced by the ratio of specification weight to overload weight but not below maneuvering speed. Again it advised caution in all flight conditions. There was discussion about pull-up speeds and the use of full deflection of control surfaces.
Source: Agricultural Aircraft Safety Review -
Agricultural Aircraft Safety Review
Seems to me that under the circumstances, the factors considered by the NZ CAA may have been applicable?