I'm not sure why you list Michael Huff here. He certainly hasn't overwhelmed the NFL so far. Roy Williams is a guy I would hate to have on my team. Cedric Griffin has been alright so far. The rest of the list, other than Hampton, are nice little players that not much was expected from and they've generally filled their roles well.
mike williams
shaun rogers
ced benson
vince young (considered retirement after a successful rookie year?)
leonard davis (really nice big oaf, but major underachiever)
ricky williams
roy williams
corey redding
quentin jammer
chris simms
frank okam
this is a list of elite UT players, either all high first round picks since Mack has been there, or guys that were expected to be high picks but faltered late in their college careers. all of these guys have made less of their careers than their talent would indicate largely because of work ethic or a sense of entitlement that they share.
While I think Vince hasn't underachieved (never thought he'd be a good QB until at least year 5), his considering retirement after his rookie year is further evidence of the shock UT players get when they leave the shelter of Mack Brown's bosom.
The list of elite UT talent to meet or exceed expectations in the NFL since Mack Brown:
fill in the blanks, because I can't think of many...
Casey Hampton,
If you compiled a similar list for Miami, USC, Florida St., Ohio St., etc..., you would see very different results. Those players, at a much higher ratio, enter the NFL more ready to play and have a much higher rate of living up to their potential.