It sounds like you are doing the right things. You should contact the office and make sure that they know it is your first choice. Ask them if they will consider a new GMAT score and if the answer is yes, you should of course try to submit a higher score. You can write a letter with an update as you have done - you can do that every once in a while if you have something legitimate to report. You can submit another letter of rec if possible to provide another perspective. For example, if you have an ex co-worker who is a student there, they could write a letter on your behalf. Showing them that you are interested and keeping them udated on your progression is important.