I don't know whether they were giving the ACT back in the days when I was a high-school junior, but if they were, I must not have been paying attention that day...
My test prep involved the following rigorous procedure:
I got up, took a shower, and went to the testing place.
I got a 1500 (780 math, 720 verbal, and I'm still kicking myself over that math score) on the SAT. I took Chemistry, Math Level 3, and English ACH tests, and got 800, 800, and 750 respectively. And 5/5 on the AP Computer Science test my sophomore year in HS.
My HS GPA wasn't too bad, but it wasn't stellar. I was accepted early decision to VaTech based more or less on my test scores alone.
5 semesters later, my academic advisor at VT was throwing my arse out of the school for, essentially, utter failure to have a noticeable GPA at all. So no, SATs and other standardized tests aren't really much of a valid predictor of college performance. At least not in my case.
-BP