Choosing index fillfactor for large, highly inserted table
The fill factor for a B-btree index is only used during initial creation of the index, and when you insert a new highest key.  So any effect of setting this won't last for long on a highly active index which is randomly inserted, as it will quickly settle in to a steady state which is independent of the starting fill factor.  If you change the fill factor and then rebuild the index, you might get a boost in performance.  But unless you can rebuild the index frequently, it wouldn't be a sustainable boost.

The fill factor for indexes, unlike for tables, is not a very generally useful knob.  If it didn't already exist and someone proposed adding it now, they might have a hard time getting it accepted.