Hmmm i don't think that's true for everyone (putting your UAI aim low will make your mark lower than what it could be), some people will work as hard as possible no matter what. A girl who was in my maths class was the only one who did all her h/w and class work, yet planned to drop all along. Some people just do their best whatever the situation.
Anyway, i'm doing 12 units, and i'd be happy with a UAI of 80+, 85+ would be even better, but, yeah my four units last year didn't go as well as planned, business def. won't count but i'll need high marks this year to cover my dance mark seeing as it scales so shit.