Bill: Thanks for the link to your blog post. What you argue there is very similar to what some scholars argue about Q, namely, that the apostles of Jesus would have recorded sayings and teachings, which then were later compiled into a single document. Your focus on Matthew is very reasonable for reasons you mention (Matthew to be a tax collector would have been educated, literate, etc.), but some scholars even argue that Matthew wrote Q!  So your theory is interesting; it'd be neat to see it worked out in the details.