illusion
The reason for the increase would seem political in that when HC Sleigh imported the aircraft for the coastwatch contract, they needed the extra weight for payload/range specifications.
Your history is a bit out.... the increased MTOW was in place well prior to HCSleigh having anything to do with coastwatch, which was not until the later '70's. Sleigh's in fact imported very few of the Shrikes, having inherited them in a takeover. They all had the STC applied at that time.
And yes, flight testing was undertaken in regard to the performance issues and the results satisfied the DCA of the day.
Coast watch was a very different environment and in fact a few Shrikes were scrapped during that period, mainly due to corrosion I recall.
I am sure the latest investigation will cover off these issues. However my issue is the article.