You need to work backwards when figuring out what a car actually makes at the crank and automatically assume a 15% drivetrain loss. This is pretty common practice.
Here, M5 Dyno link and 335 Dyno link.
Stock 335i's make around 275hp
Stock M5's make around 430hp
With a 15% drivetrain loss, the M5 makes 500hp at the crank. This means the claimed figures are accurate for the M5.
E46 M3 makes 280hp at the wheels so with a 15% loss, that's close to 333 at the crank. So the claimed figures are accurate for the M3
Using this same formula on the 335i equals 325hp at the crank based on these dyno numbers proving the factory claim of 300hp at the crank UNDERRATED.
Why is this so hard to understand?