even if you did get it to overboost by putting more resistance on the opening action of the wastegate that wouldn't really matter as boost is measures in the chargepipe, the DME would limit boost based off that measurement. once it sees overboosting you would go into limp mode and need to restart the car.
$500 for a piggyback isnt alot of money, and if you find one used, you wont lose any money when you resell it.