it does give insight into the inherent intellectual capability- and somewhat into the focus and discipline of a person.
i.e. some clever people who dont study still do quite well, and other not so clevers who work hard can do very well.
BUT, if you do crap, you are either dumb/average and slack, or work really hard and a thick head.
You dont get smarter as you get older- you learn more, but you dont necessarily retain it. Thats why IQ tests apply from age 15 onwards and dont change anty more than 5% throughout an adult's life- at least until senility.
So in summary, if your IQ indicated that you're a moron when you do your SATs, then you'll always be a moron.