"The presence of the restrictor, which actually accelerates the gases flowing through it and directs the air flow to the center of the turbocharger's compressor wheel, essentially increases the engine torque. We can roughly estimate the drop in engine power to be as high as 40% whereas the engine torque can be increased, through appropriate engine management, by around 20%"
That is a load of crap. Unless the car is operating at high speed and has a functional aerodynamic intake scoop, the only driving force causing air to flow through the restrictor is the pressure difference created by the turbo. Engine torque is proportional to mass flow, and for a given mass flow, a larger pressure difference will be required with the restrictor present. This means the turbo will be operating at a higher pressure ratio, and using more energy, creating backpressure and limiting output. Velocity of the airflow through the restrictor is irrelevant.
I think what this article is missing is this:
For a given engine, with a given maximum intake pressure limitation, there exists a certain size of turbocharger that will produce maximum peak horsepower. This configuration will have reduced low end torque due to the inefficiency of the large turbocharger at low mass flows. Let's say for argument sake on a given car that this 'optimal' turbo has a 67mm inducer.
If we then make a rule that this engine must be fitted with a 50mm restrictor near the turbocharger inlet, the 67mm turbo will go in the trash and we will fit something with a 50mm inducer, or maybe slightly larger. Now we have a smaller, faster spooling turbo which will make much more low end torque but run out of steam at high mass flow rates. We will make more low end torque, but much less peak power. For a high revving engine, this could in fact mean that peak torque is increased but peak power decreased.
In other words, the increased torque is not due to the restrictor, it is because the restrictor forces the team to fit the engine with a smaller turbo.