Steve,
have you reset the ECU and then driven the car for a few days and recorded what it is triming? I have seen cars running up to 10/12% without any trouble.
I think the O2 sensors detoriate over time so they sometimes trim unneccessary, this is one thing we do when we remap the ECU we reset the Injector K value to sync the cars O2 sensors to a known calibrated sensor it does make the car drive better we find.