Terrific pictures, Warmtoast, especially the one of the great boat coming off the step, just look at that flap area! Bob was very clear about three-mile runs on a calm day. He said his Sunderland was underpowered and overloaded with tanks filled to the brim for max endurance, hence the much resented hour-long tow to the takeoff point before starting up for an eight or nine-hour Atlantic patrol. They also carried over a ton of depth bombs and thousands of rounds of ammunition. Hence the Pegasus engines often had to be flogged at full throttle, causing wear problems if not failure.
Maybe your Seletar Sunderlands were Mk V with Twin Wasp engines producing at least 20% more power, and did they carry armament? Whatever the explanation we doff our helmets to those hardy folk who flew the wartime versions.