Shem,
I am probably noted for my long posts and even longer questions
Yes it is complicated. Have you looked at scales and/or letters in a course? I wonder if Moodle can determine competency and you are doing too much work. If I understand you: basically you set a standard on the assessment (> 80% = competent). That is like a pass fail scale.
There is way to export the underlying score in different formats that can include the raw or percentage score, I saw Letter(percentage).
Did you notice the grade boundaries and feedback for each one in quiz settings. Feedback would be a place to tell the 100% student they are both competent and don't need to take it again (see below). Consider changing the letters to say something else, ditto the boundaries. Thus you might be able to get "Competent (95.0)" in a report and tell a student in a boundary feedback "Competent, you can not improve your score.." etc".
Moodle City Hall says the basic philosophy is that when ever possible students should decide what they need in order to learn. A quiz is considered a learning activity. Thus if you allow unlimited attempts with the highest score recorded and the activity is always open, then Moodle considers even a 100% student might want to review the exam as a study guide. Consider one of the ways to shut the assessment if you don't want the 100%'ers to go back
Of course you could tweak the php code, which is not my recommendation.
I hope some experts weigh in here. MoodleDocs is down or I would give you some links instead of typing.
Chris