I think Williams will RS. I think he's a bigger version of Shump in a whole lot of ways. Very physical RB who might like contact a little too much but doesn't appear to have breakaway speed. He's a battering ram.
As you pointed out, we're very deep at RB. J-Rob is in line to be the #1 back. That 6.0 YPC for his career will come down some simply due to the increased number of carries, but he's the best pure RB we have on the roster and his hands are underrated. Not as good as LP's, but good enough. And he's got outstanding speed - one of the fastest guys on the team, supposedly. Shump will be the short yardage battering ram. If he'd learn to run to daylight instead of looking for a LB to truck, he could be special. He's going to have a short shelf life if he doesn't change his style. Word is Nick Griffin looks much better. I saw him in the M&W Game. He looked better than last season, but he didn't look 100% either. That said, he'll be a year and a half removed from the injury now, so he should be at 100% of whatever he's going to be. He'll get some short yardage carries and probably see a little time as FB. He's also the best pass blocker amongst our RBs. Then you have to factor in 2-3 carries a game for Holloway as a scatback trying to hit the HR.
Considering the depth chart, I think Williams would have to work his way up the #3 to justify burning his RS. I think he could easily pass Holloway. The question is whether he'll be able to pass one of Shump or Griff. Plus, he'll likely be the worst pass blocking RB we have. Most freshmen RBs in MS haven't really been asked to pass block in HS, so it's a skill they never really developed. Wouldn't be at all surprised for that to be the case for A-Train.
All that said, it wouldn't surprise me at all to see us use him like Shump last season. Essentially feature him in the non-con games and give him an increased role in SEC games as the season progresses, should he earn that opportunity with his play. Regardless, it'll be interesting to see how it shakes out. You can pretty much never have enough RB depth. But if it were up to me, I'd try to RS A-Train. Just my .02 cents.