Unfortunately I cannot answer this, but rather I am wondering the same thing. I do believe that your status (graduating senior, 1st year, or 2nd year) does influence the weight placed on your proposal. I'm a undergrad senior and was awarded with E/VG, E/P, E/G... yes, the middle reviewer gave me a poor because I did not explicitly define and title a section "Broader impacts".
But in their summary statements, the reviewers were overwhelmingly positive because of the fact that I had two first author pubs and was involved in a significant amount of STEM outreach. Ultimately, I think the proposal serves as a baseline (i.e. student scored X points on proposal so we'll look at the rest of the app), and that baseline threshold increases as you advance each year academically. Like the GRE, the proposal likely only sways minds in the most extreme cases such as 6 E ratings. Just my thoughts.