Slatch
Takeoff and VMC in a piston Goose was around 70 mph. Probably the turbine takes off close to the same speed. But with the increase in MTOW, Engine HP, and the engine being placed around 4 feet further forward and no changes to the rudder I doubt VMC stayed at 70mph.
I pretty much agree with what you have said except that these engines are much closer to the centerline compared with a more conventional Goose.See picture.
http://www.iflymat.org/picts/Grumman%20Goose.jpg
The modified aircraft with its more closely spaced engines doesn't generate such asymmetry despite its higher HP and there is probably substantial energizing of the rudder airflow from the new engine location. Also, there is a substantial dorsal fin.
To summarize, it looks like due consideration was made for the higher HP turbine engines and their effect on VMC.