There are more disadvantages than advantages using ITV's in a turbo application. Whilst you will have an apparent throttle response advantage off boost, in reality it's how much turbo lag you have that determines how responsive the car is to drive. This is the same regardless of throttle arrangement.

The major disadvantage is the mapping problems. This is because when using ITV's you encounter considerable difference in actual airflow against the same boost pressures at different throttle angles, meaning that you cannot map against either throttle angle, OR pressure alone. You actually have to compensate for throttle angle in addition to pressure (i.e. half throttle at 1 bar boost is not the same air flow as 1 bar WOT) which requires complex mapping.

A minor disadvantage is the fact you now have a throttle spindle in each inlet tract which acts as a restriction in comparision to the completely unrestricted tract of the single TB setup.

So all in all single TB is best for aftermarket ecu turbo setups, power and ease of mapping!