I think with 10k hours each person can be the best they can be but in the majority of cases that with be a basic competence level. I could learn enough technique to become a competent producer of pictures for instance but I still wouldn't be an artist.

It is ok to have a tough teacher if a) they are consistent, b) they are fair, c) the truly believe in your ability to improve and d) you know they are not going to give up on you.