One of the biggest misconceptions BMW enthusiasts have about fuel is that more of it equates to more power. Many people go looking for power by increasing the size (increasing the flow rate) of their fuel injectors. Installing high flow injectors by themselves will WILL NOT increase horsepower.
Increasing your engines ability to flow air WILL increase your power and when you modify your car's engine to significantly flow more air, additional fuel to compliment the increased airflow is needed to SUPPORT more power. Therefore, the only time you make more power from injector upgrades is when your engine is flowing more air than stock.
This brings us to the next point, "When do I need to change my injectors?"
BMW sized their injectors decently and you can perform simple modifications like installing a better flowing intake system or exhaust and remain within the injectors ability to provide proper fuel amounts. When you make substantial improvements that increase airflow and the factory injectors can no longer keep up, larger injectors should be used to provide the needed fuel. We generally like to begin increasing the size of a cars fuel injectors when we aim to make 25+ HP over stock.
For a simple example, people with M20 powered BMW's LOVE to upgrade to 19Lb/hr injectors for the sake of saying they have upgraded injectors. While this is fine and does not hurt anything when accompanied by a proper tune, the 19Lb/hr injectors DO NOT improve the M20's power, YET. When an M20 is starting to reach and exceed 190 HP (168 HP is stock) upgrading to a larger injector is critical and WILL help increase power. The common 19Lb/hr injector is a good choice at this point.
Fuelling does not stop there. Sometimes the need for higher flowing fuel pumps or other fuel delivery components are necessary to help maintain fuel at the injector. We like to install high flow fuel pumps when injector size is roughly double or more of the stock size. BMW, has done a good job with the fuel rail itself, but it also can be replaced with a higher performance fuel rail around the same time you upgrade your fuel pump.