Really, in my experience it still varies..I have a student who was born deaf who has a CI. She can easily switch from spoken English without sign support and ASL without voice without a problem.
Then we have an older boy who is in middle school who was born hearing but became deaf, I think around 5, who has absulotoly no oral skills.
However, the chances of recieving better benefits from implants are better with postlinguals than with prelinguals so you are right to some degree.
That's why I believe in exposing all deaf/hoh chidlren to ASL along with spoken language cuz we never know who will benefit from whatever hearing they have. Heck, I can speak and read lips better than some HOH people despite being born with a profound deafness in both ears.