my son go to speech therapy and he does not have ASL. And I don't teach him about vocabulary or anything like that, and he have a high score reading and writing dispite him being with me all the time (husband always working)
Most CODAs interprets, even off from TV, so I don't know how in the world ASL affect speaking and writing ability when they are heavily involve in it anyway. I've seen my mother's cousin interpreting everything for her own mother, but she is happier in ASL than she is in English.
But on the other hand, my son learned my speaking ability and have to have speech therapy to undo that.
