I agree that they will cope arround 14.7 (lambda 1), above and below is a bit subjective.
My point is that they give suffient detail to ensure that the engine is not running lean, I personally have had quite a good result from these sensors.
I find that they will give suffient detail to 13:1 a/f ratio which is fine for me.
I`m looking into making a wideband sensor controller, but personally believe that it is best to tune on a rolling road and use in car sensors as a backup mechanism rather than tuning.
Thanks
Darren