It seems to me that your case is pretty strong. As you said, you might miss some key points in your cover letter.
In my opinion, FOR YOUR CASE (I mean for your case only), a very well written RFE reply letter is MORE important than more letters, since you already have quite a few letters. The key point for you is how to present your qualification very well.
For more information, you can email me at fsa2000@gmail.com