Maybe is not the position, maybe is just the skill distribution, but I have never seen an Allstar SF who improved when he was more than 45-48k. I trained a PG that reached the limits when he was 65k, and now Im training one with a virtual wage of 58-61k and keeps improving.
I have seen also some SF capped when they werent even 40k.
If someone finds an SF of 50k that keeps improving fastly I would be happy to see it.
About the sublevels, Im sure that the potential has sublevels, everything - related with skills - in this game has them.