Tried the stock limits for AFV with midpoint voltage on O2 sensor at 0.5V and it had an edgy-ness to the throttle response that wasn't there with the 107% locked AFV.
Went back to upper and lower limit AFV of 107%, and kept the 0.5V midpoint voltage and its sweet again.
Funny thing though, the CEL came on a few times with "Rear O2 sensor always rich". Went off again after a few miles, but not ideal. It didn't come on when I first raised the midpoint voltage a few days ago. Maybe not enough miles to 'learn'?
Will lower the AFV% progressively to see if I can make the CEL stay off.
Sure runs sweet now though by comparison with stock set-up