It looks like the code to handle that was stripped out finally in May, 2011. It conflicted with the new way of drawing and it's not clear if we can influence the threshold in the same way.
It might be best to ask / file a bug against WebKit which drives Safari's rendering.
To me, the new code works so much better, I don't need the speedup or the control.
See http://www.ichemlabs.com/618 for details, but here is the nice chemical rendering that shows the differences between the old and new default behavior. Full size, it's impressive how much better the rendering is at the same (or better) rendering speed.