Not knowing the potential and not having any potential are two different things. Everybody has a limit on their potential. The players in the video game should have limited potential. We, as players of the game should just not know what that exact potential is. Our scouts and Coaching staff should give us there best guess.
As far as Cassel exceeding his potential in stats. Why did he exceed that potential? Is it the system he's in, the coaching, are we controlling him so that he plays much better? Where the coaching staff and scouts wrong about his potential? Was his potential always higher and we got a more accurate accesment of his potential after some playing time?
Matt's progression should not be determined by what kind of stats we can get him. Matt's potential, coaching, mentoring of other players, work ethic, practice, playing experience and so on should determine his progression.
The only way you could get me to subscribe to stat-based progression is if I could control all 55 players on all teams so the progresion ends up fair (probably not so reasonable or balanced) for each and every player in the league. Otherwise skewing the progression of certain players pretty much invalidates a real franchise.
You also have the problem of counting on the game giving each position a fair and resonable amount of stats. There was one year that I got tired of drafting RE's. They never got sacks. The LE always got the sacks. The LE got all kinds of progression and the RE would always turn to doo-doo. I moved my stud LE to RE and my crap RE to LE and the stud LE turned to crap and the crap RE started to progess.
Stats based progression is just a lazy, crappy system to use.