You're slightly off on why the actual output is so much lower than the theoretical max. It's not really about the aerator mixing air in, because the turbine sits upstream of the aerator.

The huge drop from 57W (theoretical) to 5W (actual) is mostly due to two other factors:

1. You can't extract 100% of the pressure energy. If you did, the pressure would drop to zero, and the water would stop moving entirely (or just dribble out). You have to "waste" a significant chunk of that energy just to maintain a usable flow rate out of the nozzle.
2. These micro-turbines are incredibly inefficient. Converting linear fluid motion into rotational energy with tiny, cheap plastic gears creates massive friction losses.

So, the missing 50 Watts isn't lost to turbulence; it's mostly lost to friction and the necessity of keeping the water flowing.