Fast951 - I guess it depends on which system you are talking about. I have worked on only two, Bosch and Megasquirt. Megasquirt does not even have a WOT switch implementation and simply uses a 2D map for the fuel calculations all the time - even under boost. Load is simply determined by intake manifold pressure. However, the Bosch Motronic code on the 3.2 and 3.6 has dedicated WOT tables and the WOT switch.
There are several reasons why I believe (and don't know for sure) that the Bosch system does not use the metering reading at WOT:
1. The WOT table in the Bosch system is 1-dimensional: fuel = f(RPM) while the part throttle tables are 2D: fuel=(RPM, load). The axis definition on the Bosch code is pretty straight-forward. Calculating the bin labels shows that the 2D map load axis covers a range from 10% to 58%. See an example here of a 964 Motronic map:
2. If the Bosch Motronic was to use metering readings beyond the 58% level they would have extended that 2D map.
3. I wonder how the DME would use information from the metering device once we it is past the 58% level. Maybe there is another part in the algorithm where the metering output flows in as a linear factor. While I don't know this for certain I strongly doubt it. It would make remapping a car from a linear to a logarithmic metering device nearly impossible without re-writing entire code segments in machine language. The RPM information should be sufficient to determine how much fuel is needed.
4. Thinking back to CIS here the metering device has no information about engine speed and thus has to rely entirely on the CIS sensor plate to determine fuel quantities. My conclusion is that one or the other is good enough at WOT. This is why the WOT switch was implemented. It makes tuning for performance much easier.
It is my understanding that the Bosch unit pretty much uses a speed-density algorithm (metered air, RPM, pressure, intake temp.) for part throttle and reverts to an Alpha-N algorithm (throttle position, RPM, intake temp.)
I guess the ultimate answer would be to run a car on the dyno and simulate a low AFM reading while the WOT switch is engaged. I wonder if the mixture would change......
Cheers,
Ingo