according to this article, learning to read for a deaf person is comparable to someone that can hear learning to read a foreign language. I would imagine it would be like me learning to read chinese, they would just be symbols with no phonetic attachment. That 50% number seems pretty high, not sure I buy that.
I'm not satisfied with that article, but it does bring up some interesting points. I know that among learners who can hear, writing and learning how to spell in a target language is pretty crucial for pronunciation, at least that's what the current literature says. I wonder if part of it is because signing is a much more dynamic (in terms of sheer physical movement) system than speaking. People have different preferences for learning and recalling information, but kinetic memory is very, very strong. If that's your primary experience, then I think it would be very difficult to learn to remember all the rules and associated stuff that goes with communicating in grammatically correct written English.