MWERKS dyno'd a stock 135i and got 280hp and 295ft-lbs AT THE REAR WHEELS.
I've heard approximately 20% of the power from the engine to the rear wheels is lost to the drivetrain. Based on MWERKS numbers and 20% loss, the 135i puts out 350hp and 369 ft-lbs at the flywheel.
350/369 is a stretch; but I wouldn't be suprised if 300/300 is underrated considering the car does 0-60 in 4.8 and the 1/4 mi in 13.3.