Sounds like a reasonable analysis, although I think you'll see a bit of disagreement with your subjective categorizations. I would have liked to see a fifth category, "Average", since there are a number of guys you put in "Above Average" that I think are generous, such as Culpepper, Pennington, Locker, and Ponder.
But yeah, a 50% hit rate isn't that bad, all things considered. Part of it also has to do with guys who were drafted in the first round who probably shouldn't have been (Losman, Tebow, etc).