Thanks. Luckily you don't have to worry about that with the 03 WRX. In the 02-03 WRX they used throttle positions almost exclusively to govern the OL/CL switchover. At anything less than 70% throttle, your ECU runs in closed loop and switches to open loop above 70%. This is fine with the factory ECU because it also limits boost to wastegate pressure until you pass that 70% throttle mark.

You do have to be careful if you try to run any type of aftermarket boost control without tuning the ECU for it. Most aftermarket boost control systems will be able to bring in almost full boost at much less than 70% throttle which will cause a similar lean condition which will result in knock and possible engine damage.

Back when I first started playing with Subaru's, I actually made a little passive electronic device that watched boost levels and simulated an appropriate throttle signal to the ECU to force it into open loop mode based on boost levels rather than simply throttle position. This allowed us to run aftermarket boost control with greatly increased response without causing a dangerously lean condition. I actually designed and produced this as a product for a couple years back in 03-04 and sometimes still get requests for it.

Now that we have good access to the ECU parameters that govern the OL/CL switchover, that type of device is obsolete. The general rule now is that you can run just about any boost control strategy as long as you tune the ECU to properly handle the boost profile.

Thanks
-- Ed