Just to make this more clear here is the answer:
You said if you had three 18 year olds with these potentials:
1. 6th man
2. All Star
3. Superstar
Your question was this:
If I trained them all equally, will the higher potential players train faster? Or will they all train at the same pace until they reach their potential ceilings?
1. No, the players will all train equally as potential DOES NOT effect the training rate of players. What would effect their training rate is HEIGHT. Shorters players train outside stuff (Jump Shot, Jump Range, Outside Defence) faster. Taller Players train their Inside stuff (Inside Shot, Inside Defence, Rebounding, Shot Blocking) faster.
Hope this helps :)