How about his numbers against UCF, which both obscure quarterbacks you listed also played against
Hackenberg - 21/28 (75%) for 262 yards, 1 TD, 0 INT
O'Korn - 24/46 (52%) for 231 yards, 1 TD, 2 INT
Walker - 26/44 (59%) for 382 yards, 4 TD, 1 INT
Another true freshman QB also played against UCF.
Burcham - 24/34 (70.6%) for 222 yards 1, TD, 0 INT.
It's also worth mentioning that Hackenberg did it with significantly fewer scholarship players. He probably did about as well as an 18 year old true freshmen could have considering his schedule and roster limitations.
About Gregory, if he has another 10 sack season he'll more than likely be a top 10 overall pick. No reason to stick around if that's the case.