Here is something that I started to developed that you might be interested in; It was modelled on a character's Original and Potential Key Attributes and was inspired by Role Master 1e.
I like that idea to a point, George. I think training to a degree has somethng to do with increasing stats. For example ballerinas are usually graceful, not clumsy. Their is probably some natural selection that plays a role here, but so does their training.
My personal preference is maxing out increases somewhere in the range of if you roll a 50 on the maximum potential. In other words, with a starting dex of 3 for instance, one cannot achieve a greater than 11 in that attribute.
I would also think that the stats cannot be improved unless something was done as "practice" for that particular stat. Probably the simplest way of saying this is that if a stat is in a skillbase someone uses, that stat can be increased.
The other thing to - if this is something that the general population uses, average stats are no longer 10-11 - they would be in the 16 or so range, which changes things I think drmatically.