HLH Fibonacci extensions can be powerful enough by themselves to mark a turn. Several good "standard" Fibonacci highs have been exceeded in this (maybe) topping process as market forces take it to the limit one more time in Price, Time, and Sentiment. This one may be more potent given the 9/3-9/4 window and tape "tells".
(No one is talking about the 1929 analogue just yet.)
Now to the Phi:
The 4/2/12 high at 1422 was of clearly visible importance, as can be seen on the chart, and it has anchored other turns. 10/9/13 has stood out as a crucial inflection point in e-wave and day-counting. Its low was 1647.
SPX 1422 -> SPX 1647 = 225
225 X 1.618 = 364
SPX 1647 + 364 = SPX 2011