My son was too late to apply for SLE by the time he decided he wanted to attend a service academy, and therefore never did it. He was assured by many at WP that SLE doesn't help or hurt a candidates application.
If you want a predictor of your chance at admission to WP, I would look to two things; the class profile of prior WP classes, and the relative competitiveness of your congressional district and your state. You want to be above the curve nationally, but most important, you want to be at the top in your district, because that is your competition. The problem with that is that it is difficult to determine the relative strength of your in-district competition.
Here is a secret (and don't tell anyone else): attend a WP admissions briefing, which are conducted daily I believe. Make sure it is far enough down the road that most if not all of the applications are already submitted. At the end of the briefing and tour, ask to speak with the RC who is on duty that day (who probably conducted the briefing). That may or may not be the Regional Commander for your region (you could theoretically call and ask, if you wanted to plan your visit on a day your RC is there). When you meet, ask to go over your application, and ask what you need to be at the top of your district. We did this twice, and both times, with two different RC's, we got a very good analysis of the competition, including a lot of specifics about the person or two who made up the real bonafide competition. My son was also told exactly what scores and other things he needed to achieve in order to beat the others, including ACT, CFA, and athletic accomplishments.
WP is way more up front about that than USNA or USAFA, and the RC's road mapped everything very clearly. The briefing and tour were excellent too.