[Lorenfb]

The actual torque changes are only about 3% over the AFR range of about 11.8 to 14.7.

References:

"Automotive Handbook - Second Addition", Bosch, 1986, pg 439, ISBN 0-89883-518-C
"Automotive Handbook - Third Addition", Bosch, 1993, pg 428, ISBN 1-56091-372-X
"Automotive Handbook - Seventh Addition", Bosch, 2007, pg 641, ISBN 13-978-0-8376-1540-Q

These data provide additional support to the OP which stated that once the AFR is within
a few points of the maximum torque, insignificant torque improvements are gained
compared to small changes in the ignition timing which have a significant effect on torque.

Again, performance tuning of a stock engine, e.g. 911 3.2, in reality is just a simple effort
of increasing the timing advance over what Porsche had determined as appropriate,
given the lack of knock control.