Since it is NSC, it's not a big surprise to get a RFE.
You submitted only 6 letters. I think you need at least
4-5 more independent letters from big guys like
managers in companies, directors in national labs or head in your professional association. I believe you have submitted all possible evidence you had. Therefore, this is the only thing you can do. Recently most people got a RFE. However, don't worry. Your RFE is not the worst. The following forum has very nice discussions. Please check with it if you have not done it.
http://immigrationportal.com/showthread.php?t=181843