All of the SAs are becoming more competitive due to the same or more applications and few slots. Thus, it is important that each of every part of your application be as stellar as possible.
USNA Admissions has said that, if you don't max out on the CFA, you should consider retaking it. Obviously, you need to exercise some common sense . . . if you have done extremely well (based on the max scores), you're probably ok. However, if you have some weak areas, you're giving USNA a reason to pick someone else rather than you.
Clearly some candidates are admitted with less than stellar CFAs. However, do you want to take that chance?
As for NASS -- if you bomb the CFA and also have trouble keeping up in general runs, mock Sea Trials, etc.), that could be a negative. If you do OK on the CFA and otherwise do well during the week, a less than stellar CFA won't hurt and you can always (and should) retake it in the fall.