Strange question - 'what is highest THC level in plant'
Science is a theory of a descriptions to categories the world around us. Once it became clear that in fact THC numbers did not reflect the experience of the high then science had to look elsewhere for explanations. The terpenes and other micro chemicals are now considered to have a major effect on the high as will CBD levels(not CBN! very different cowfish altogether).
Once these are understood it will be clear that there is more to the high than this and the scientists will look closer and there will be another level of theory and explanation as to the micro micro components of our smokes.
At a recent music session with friends I laid out a selection of sativas to be enjoyed. All very strong. At the end of the night everyone had a favorite, all different and the views changed quite dramatically as to the strengths and effects of the different strains. To have correct interpretation of high then the body chemistry of the toker would also have to be scientifically mapped in.
The reason we have such strong smokes today is the boys in the fields for the last few thousand years in Thailand, Nepal ,etc all tried a few smokes after harvest and found the ones that turned them on and grew those seeds next year.
Smoke and enjoy, leave the figures to government statisticians!