So on my interview day they told us that in order to receive an interview your application had to first get the "thumbs up" from two people on the admissions committee. If you got one "thumbs up" and "one thumbs down" your application went to a third member who gave the final decision. So in essence your application may only be viewed by up to three people pre-interview. The post-interview decision making process was more complicated.
As for LOI, I'm not really sure if they are helpful since the secondary left an open ended question at the end to say essentially whatever you thought would be helpful. There is also the argument that if you applied and completed the secondary you have already indicated your intent to that school. I heard no indication that they are added to your file, but also did not hear they are ignored. If it makes you feel better go for it, but I wouldn't count on it having any real impact. I remember looking at past threads and it appeared that post-interview LOI's or updates were definitely not considered.