I've got a PF, James Johnson, who plays for my Le Moyne team in Crum. I recruited him in season 72 and redshirted him; when I initially recruited him, his overall rating was 456, and by the end of his redshirt season, he was at 538 - an 82 point gain during the redshirt season.
During the summer/offseason he gained another 18 points, so by the time he finally took to the court in his first real game in season 73, he was up at 556 - 100 point gain between the time I recruited him/redshirted him and the time he finally got to play. 93 WE to begin with.
http://www.whatifsports.com/hd/PlayerProfile/RatingsHistory.aspx?tid=8252&pid=2927386
I'm sure there are guys who have seen more significant growth than that, but I was/am pretty pleased.