SAT Subject Tests - Necessity and timing

[skieurope]

However, two year courses (e.g. two year IB HL courses, or AP physics 1 + AP physics 2) may not cover all of the topics of the SAT subject test after just the first year.

It’s not a question of may not; it absolutely does not.

The students I know who took the bio subject test after 8th grade high school biology, all did much better after AP Bio.

Which may as much a function of the maturity of the 8th grader as the preparation received. In any case, AP is not needed for the Subject Test, although some students may feels more comfortable with the additional preparation. However, depending upon the school and/or the teacher, there is no guarantee that either course will adequately prepare the students.