So, your concern is noted. And while I appreciate that you have a good deal of knowledge with numbers, what you could help more with is a solution to the possible issue of sub-levels on the cap. For example: how to prove they exist (or not) and if they do, how to model appropriately.
The trick in figuring out potential potential sublevels is making sure you don't confuse them with skill sublevels. My best guess would be using the "current" salary level (or DMI in proficient game shape) as a regressor to control for unobserved sublevels in skills. If the per-position regressions produce significant differences between predicted and observed values of potential (or high error terms), this indicates differences within potential levels.
The problem with this approach is a certain collinearity on the right-hand side, since salary is also a product of skill levels. This may be addressed by running a regressin using just the salary levels (this will test against the assumption that the potential "value" and salary are linearly proportional).
This is mostly brainstorming, and I am sure there are at least a handful of users here who know how to work data, and can toss in their own ideas (Coco?).
All in all, it's not that I have concerns, per se. I do think that it is great that someone is willing to do this type of "reverse-engineering", since it's long overdue. I don't mean to come through as condescending, just trying to provide some pointers about data work, since I do have some experience there.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."