n00b tech 101
The converter would never be locked at full throttle anyway. The “torque converter” does just that. The slip causes the RPM of the engine to be higher than that of the input shaft of the trans converting RPM into torque. Thus why a high stall converter car will always dyno with low HP and high torque. HP is torque multiplied by RPM. The slip of the converter acts like a lower gear ratio reducing output RPM and multiplying the torque reversing the equation.
In all if this were a manual it probably would have dynoed closer to 400/450 than 300/550
The advantage of the auto with such an inefficient torque converter is that it will keep the engine in its most optimal range much like a CVT.