the restrictor limits the amount of air that can enter the turbo so caps the power, I still have it on so I have a way of fitting an air filter, you could run without it for full power or get it mapped with it on for big torque.
In use it actually drives very well with one on, linear in power delivery like an N/A I would like to see a power graph to compare as its more driveable with it on if not as much PSI