I like the idea of slightly increasing the potential for your drafties if you keep training them. It's like a boost to the draft, but only if you train your own players. Leaving apart medium and high potential players(4-5 stars), it could increase the number of 3 star players that actually get training.
How much more potential? 1 level up. That extra level is only shown to you.
I think the players you used as an example aren't the best. I am thinking more about late second rounds, undrafted players that make it to the NBA like:
Udonis Haslem
Raja Bell
Jose Calderón
Avery Johnson
Brad Miller
Bruce Bowen
Ben Wallace
Gary Neal
Jeremy Lin
Late second draft:
Isaiah Thomas (60th pick, 2011)
Danny Green (46th pick, 2009)
Patty Mills (55th pick, 2009)
Goran Dragic (45th pick, 2008)
Marc Gasol (48th pick, 2007)
Paul Millsap (47th pick, 2006)
Marcin Gortat (57th pick, 2005)
Kyle Korver (51st pick, 2003)
Luis Scola (56th pick, 2002)
Manu Ginobili (57th pick, 1999)
Steve Kerr (50th pick, 1988)