Dan, et al.
If I in any way diminished, downplayed or made light of your issues or the documented issues of stability that has been noted by others then I very humbly apologize. That was not my intention.
I admire your work, enjoy offering suggestions to solving the issue and look forward to you completing your experiments, you have vast more experience at the road handling aspects of of two wheeled vehicles than I will ever have despite my having restored several bikes and riding all kinds of marques for 35 years.
My intention was to make the point that most of us casual non-stress (fair weather and sunny days) riders do not experience the high speed instability that you and a few others have seen.
I do wholeheartedly agree that based on the number of bikes on this forum and the number of folks complaining of similar issues the statistical sampling is unacceptable.
That being said, I do feel that the issue is just far enough outside of the norm that if it exhibits itself the the first thing to do is to check and recheck the intended factory settings. IMHO that will cure a measurable percentage of the bikes with the issue. Not all of them, certainly not yours. Also bear in mind that some folks are far more sensitive to such issues. You are an experienced canyon runner, I am not. In my world if I were to pick instability at high speed, I would simply back off on the throttle until it subsides.. In your word, you feel it and it bothers you. It's gotta' be fixed.
Also note that there are a lot of new dealers now selling this marque. I have little confidence that my dealer, (although a great guy to do business with) would be qualified to anticipate such an issue. I don't know if they really know how to set up an Enfield. I had the perception that they did not participate in any factory training at the mechanic level. So how many bikes are exhibiting these symptoms because the inexperienced dealer does not know what to watch out for upon assembly?
Any Case I was attempting to suggest a logical starting point for the OP, Based on what I perceived from his post he is not as experienced as you are and I feel that anytime a handling issue raises its head then you need to start with the basics, and that is chassis, tires, bearings etc. To most folks that should start with a trip to the dealer.
If I offended then I am sorry.