Quick and dirty... On a stock ecu the safc interupts the maf voltage signal on its way to the ecu and decreases the voltage. Decreased voltage equals decreased airflow. It is doing this to trick the ecu into thinking less airflow is entering the engine. In turn the ecu will compute a shorter pulse width to compensate for a larger fuel injector. This allows the user to achieve the desired air fuel ratio.
The bad part is that airfuel ratio is only a small small fraction of a "tune". In fact timing is substantially more important to making power and preventing knock. So, when the SAFC makes these corrections to the maf signal to achieve the desired shorter pulse width outputs it is essentially tricking the ecu into "thinking" less air is entering the engine. Anyone familiar with the basics of tuning will tell you less air/less load requires MORE timing. The lookup values the ecu is referencing to come up with shorter pulse widths also correlates to more advanced portions of the timing map. The exact amount of timing you were running is unknown and I can only speculate. If for example the ecu was referencing the third column from the right on the stock timing map you would be running ~17* around 5000rpm. It is my opinion that with 91 octance gas this is definitely too much timing. When using an SAFC to correct for fuel you are essentially flying blind on the timing map.
*The only minor exception to the rule that the SAFC is not wise is if you have a mail order rom tune that gets you in the ballpark and only minor minor corrections are made.
I highly recommend an ecutalk (
http://www.ecutalk.com/) to monitor your actual timing. It is arguably expensive but I have found it extremely useful.