The star system is not a gauge of talent. It is a gauge of immediate impact to a team. A 5 star player can sign out of high school and make a direct impact on any team. A 1 star player is projected to need years of coaching to make an impact... potentially his junior or senior year.
I think it depends on the source. Here's Rivals.com's explanation of star rankings:
[*]6.1 Franchise Player; considered one of the elite prospects in the country, generally among the nation's top 25 players overall; deemed to have excellent pro potential; high-major prospect
[*]6.0-5.8 All-American Candidate; high-major prospect; considered one of the nation's top 200 prospects; deemed to have pro potential and ability to make an impact on college team
[*]5.7-5.5 All-Region Selection; considered among the region's top prospects and among the top 500 or so prospects in the country; high-to-mid-major prospect; deemed to have pro potential and ability to make an impact on college team
[*]5.4-5.0 Division I prospect; considered a mid-major prospect; deemed to have limited pro potential but definite Division I prospect; may be more of a role player
[*]4.9 Sleeper; no Rivals.com expert knew much, if anything, about this player; a prospect that only a college coach really knew about
Players can be ranked by pro potential, and I think Tyler Love was an example of that last year. Most analysts and experts believed he needed a year to put weight on and get stronger, but his potential was up there.
But, yeah, the 5-stars have often already "maxed out". This is especially true for offensive linemen, as many guys come out of HS both technically and physically behind, then develop in college in the S&C programs...
4 and 5 stars are most likely to imact a team early, while many of your 2 and 3 star prospects take a few years. The exception to this rule, is that many of your 4 and 5 star prospects play for teams with fewer opportunities early on because of depth. Your 2 and 3 stars often sign with teams like Vandy, MS State, etc., that have more opportunities for freshmen. Look at the Freshman All-American teams vs. the All-American teams. The Freshmen teams are littered with 2 and 3 star players from Akron, Central Florida, and Houston, while the All-American teams are more often dominated by 4 and 5 star players from Texas, Florida, and Alabama.
I've looked at the data, and from 2003-2006, about 30-35% of each position in the Rivals100 made the pros. The two exceptions:
ATH - about 50%
OL - about 10%
Kind of rambled here, so sorry...