Conventional school means of assessment definitely are an obstacle for radical acceleration. My sib's experience was certainly not the norm for anyone, and was greatly facilitated by interested faculty members allied with parents, as well as the choice of a math-focused major. And I should note that wet labs also were pushed off as late as possible, to minimize safety issues from differences in physical stature. But the fact that the overall arrangement was allowed definitely worked to the advantage of my sibling, who would otherwise have had an even more trying educational experience. It's unfortunate that more students do not have access to accommodations for relatively "simple," but pervasive, obstacles like writing. If there were some way to write accommodations for asynchrony due to giftedness, in the same way that there are post-secondary 504s for asychrony due to LDs, it would really help kids in this situation.
I also don't think we will be able to swing a similar arrangement for our own writing-discrepant child, in the absence of special faculty support.
And as to placement criteria, as we are homeschooling, until each of our children exhausts the secondary curriculum, I have targeted 80-90% (accuracy on end-of-course assessments, not percentile), as the upper half of the ZPD, on a subject-by-subject, year-by-year basis.
