I don't see anything that would indicate lessening back pressure would be noticed by the PCU. There are 2 wideband O2 sensors; one installed in the Cat (Downstream Sensor) and one installed before Cat; possibly on the downtube leading to the cat (header?). This is the upstream O2 sensor. Their function is to send the info for the PCM to set the injector pulse width according to measured oxygen levels measured before and in the cat.
Both of these sensors are far upstream from the muffler. The sensors are wideband, with heaters to stabilize them, so I don't think higher flow would effectively "out-run" the speed in which the O2 sensors can send info to the PCM.
I have little experience with turbos, so verify this with someone who is more knowledgeable on them before accepting it as fact. This is just a commentary on what the service manual shows in the exhaust system.
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