Garza went from an average post to an all-american in one season. Grill can certainly improve from bad to solid.
LOL, you think that is "typical" development? Sure you can pull a few examples of players improving exponentially. I can also give you 50 examples of typical development for every 1 Allen Iverson or Luka Garza type development.
For instance, Conditt was meh last year, and is meh this year. Lewis has been pretty non existent for 3 years and he was a 4* with a good offer list. Jacobson has been pretty much the same player his entire career. Nixon has regressed at ISU. Young has been the same player for 4 years now. Griffin was a 4* with good offers who hasn't done anything for 2 years.
What Prohm player has really exploded out with great development if they didn't come in gang busters as a Fr. (like THT and Wiggington)? Sure anything is possible, Grill could be the first, but the odds say he won't be.