First off, don't tell your sponsor what to say. They should speak from their heart according to their knowledge of you and experience with you and nothing else. They can tell the judge how long they've known/worked with you; what step you are currently working on and have completed and their opinion as to how you are doing in AA. This is an honesty program and that's what the judge wants to hear and what your sponsor should say. Anything else is ego speaking.
You may NOT write that letter. It violates several Traditions of AA. You MAY write a letter saying how long you have know the person and your perception, i.e., dependable, trustworthy,... This does not say how you know the person. State your position in the community if it will reflect positively on the person. Think of yourself as a character witness/reference.