Actually, we really don't know anything about the players on this team.
Stairs has made a substantial improvement in player performance, and while it's taken a while for some players, the team is a much better hitting team than it was last year or this spring. And since the veterans have flopped or been injured, that improvement is primarily the "kids." How much can they improve? Has Franco turned it around? Are Altherr and Williams for real? Stay tuned!
Rupp now has a full season worth of ABs (587) over the last two seasons:
.247/.310/.447/.757
If he doesn't improve at all, that's still pretty solid for a starting catcher. He's a 2.4 WAR (BR) player over that stretch.
We have no clue whether Alfaro can hit ML pitching, he has a .660 OPS in AAA ball at age 24. He has a fundamental flaw in plate discipline, and hasn't shown the ability/willingness to adjust that Williams did this year.
Knapp is an even tougher call, he's putting up similar numbers in Philly so far as he did in Lehigh last year at age 24 (.720 OPS but .124 ISO).
What makes him weird is he hit for power in Lakewood, not in Clearwater, power in Reading, not in Lehigh or Philly.
As as switchhitter, he has extra value as a backup catcher, and he can play multiple positions.
Grullon is the hardest to project, hasn't hit much but was under age at every stop, Lakewood at 19, repeated at 20, Clearwater at 21, now he's in Reading.
He's probably the best defensive catcher at any level right now (arm is rated at 80), so the question is the hit tool, but he's young enough to spend 2-3 years learning how to hit.