It has been a while since I have done an injector test on an n54/n55, but for the n63 there is a span of time during the intitial test where your crank the engine and allow it to start, then immediately shut it down. The tester will monitor the fuel pressures of each rail to see if there is a fuel pressure drop that is outside specification.
How does the car start when it is cold? A bit of a stumble with no fault codes is a good indication that there is an issue brewing.
Also, do you do a lot of short drives? And when was the last time that the spark plugs were replaced? This can affect cold start/ cause a bit of fuel wash every time you start the car as well.