Correct me if I'm wrong, it's just an hypothesis:
At the moment all the "tune guys" are working on the Subaru ECU, which manage the indirect injectors, while the Toyota DI ECU is still untouched, am I correct?
I think the dip in the torque depends on a A/F ratio too lean, or it's connectect to the variable timing system.
If you look as this dyno chart (it's from our full exhaust, notice the lambda curve) you see the peak of the O2 sensor is close to the torque dip:
