Depends on the turbo...

Turbo chargers have something called an "efficiency range". The Efficiency range is a gauge of how efficiently the turbo is turning exhaust gas rotating the turbine into airflow coming from the compressor wheel offset by how much heat the turbo is producing at a given turbine RPM.

When a turbo is pushed beyond its efficiency range, the amount of air flowing past the compressor wheel becomes less and less for every unit of gas going past the exhaust turbine wheel. The faster a turbo spins, the more heat is passed onto the air as a byproduct of the internal friction from the moving internal components. As the turbo is pushed beyond it efficiency range, it continues to produce more and more heat, but flowing less and less air per unit of exhaust gas.

This excess heat applied repeatedly, causes stress in the form of heat and friction on the internal components, as they are operating outside of thier capacity to lubricate and support the rotating compressor wheel. This reduces the overall life of the turbo and can cause failure in the turbine bearings after prolonged use outside the efficiancy range of the turbo. The oil coming from the engine needs to run at a certain temperature. If the heat from the turbo pushes the oil temperature beyond its normal operating range, the oil begins to break down and it capacity to lubricate is lessened.

If you want to flow more air, outside of your current turbo's efficiency range, you must consider upgrading to a larger turbo. A Larger turbo will be able to flow more air at a givien turbine RPM compared to the smaller turbo. Less turbine RPM equals less heat. Therefore, a larger turbo, is more efficent and has a higher efficiency range.

For Example:

The Standard TD04 that comes with a 2006 WRX normally operarates at around 13.5psi standard. This turbo, being quite small, is already near the end of its effciency range. You could increase the boost pressure to 16psi or 20psi quite easily. However, at 20psi, this turbo is not prducing very much more air flow than if it were set at 16psi. It is however producing far more heat. Running a td04 at 20psi would cause undue stress on the internal components due to heat and shorten its overall life considerably.

This is why the STi comes standard with a larger turbo. The Sti is designed to run close to 20psi stock standard. With the upgraded vf43(I think) It is able to flow more air with less heat at 20psi than a td04 at 20psi. This in turn increases the reliabilty of the turbo and allows Subaru to sell the STi without having to worry about turbos failing during the normal use of its product by one its customers.