You need to use plain English for laymen to be able to understand how useful your work/research in terms of national interest.
For example, if you work in cancer area, if you say your work is very useful to understand the mechanism of carcinogenisis. It is not problem for scientists to understand but not good for laymen. For laymen to understand, you just need to say, your work is helpful to detect, diagonosis, prevent or cure cancers. Then they will understand how important your work is.
Check out the following posts and collect more evidence or any new evidence you have in addition to more recom letters.
Two or three pages are enough for the recommendation letters. 6-10 pages for RFE response letter should be ok.
Good luck.
http://web.wenxuecity.com/BBSView.php?SubID=immigration&MsgID=100444
http://web.wenxuecity.com/BBSView.php?SubID=immigration&MsgID=99560