Pretty much regardless of the boost level, you will still see "rich" because the ECU calculates the air volume from the AFM, and matches the fuel using its internal air-fuel maps. Unless you have a turbo that can pull more air than the stock AFM can measure, you should never see the AFR go "lean" at WOT (on a stock ECU).
The change in voltage of the stock O2 sensor is extremely rapid around the stoichiometric point. In a matter of perhaps 0.1 voltage units (maybe even less), the AFR will go from perhaps 13:1 to perhaps 17:1, totally missing 14.7:1. This is why you will see the gauge dance around the stoichiometric point - the change is so rapid that the ECU cannot make fuel corrections quickly enough to maintain a stoichiometric mixture.
So, as mad082 stated, if you try to tune using a AFR gauge, by the time you have got enough boost that the engine runs lean with that boost, you will be on the wrong side of stoichiometric, and will very quickly destroy the engine.