It was confusing for me! Here is what I learned and why I ended up using the SERT on my 07.
The ECM is only using the O2 sensor feedback signal when the system is in closed loop mode. Closed loop mode occurs only under certain conditions which are light load, steady speed part throttle, constant mode(where most of us spend majority of time on the highway). It is only after these conditions that the EFI will go into closed loop mode and the ECM will adjust until the O2 crossover point is in the 14.7:1 A/F range. The beauty of the race tuner is that you can adjust the bias (map) to have the closed loop AFR be say 14.2:1 using the feedback from the narrowband sensors (hence the +/-0.5 afr adjustment). Otherwise you are using the programmed map in the ecm or with the PC3, the piggyback adjustments to the base map.
Kojak,
The problem I have with this theory is that the narrowband sensors work like a light switch, not a dimmer. They only tell the ECM yes or no, nothing in between. The only message the ECM can get is rich for AFR's under 14.7, and lean for AFR's over 14.7, or some other setting, but the point here is that this crossover point is fixed. It can't move.
When the ECM goes into closed loop mode, the ECM keeps checking the feedback voltage from the O2 sensor to see if the sensor says rich or lean. The ECM then starts removing fuel from the injectors until the sensor switches from rich to lean, or true to false, or whatever. The point here is that this switch always occurs at the same point.
In order for it to work as you have described above, the SERT would have to change the crossover point of the sensor, so they would switch from rich to lean at a lower AFR. I don't think that is happening. I don't think the SERT is capable of altering the sensor's crossover point, which means on the road, in closed loop mode, you will wind up right back at the factory lean / EPA compliant 14.7.
In my mind, the use of a narrowband sensor in a closed loop system will always wind up at the same AFR. Otherwise, the sensor is being ignored, and you are not in closed loop mode.
If you could replace the stock narrowband sensor with a different sensor with a lower AFR crossover, then you could use the part-time closed loop system and get a richer condition, but the stock sensors will always toggle at the same point.
I could be wrong, and hope someone will straighten me out if I am, but this sounds logical to me.
Chief