Quad copter speed limited at 185 km/h?

Hi, I have built a quadcopter that I hope will do 300 km/h. It accelerates very quickly to 185 km/h, 50 m/s, then the controller cuts back the motors despite full throttle from my transmitter. I have done a number of runs and it consistently cuts back motor output at 50 m/s. I am flying in Stabilize mode, pitch nose down at 80 degrees. ArduCopter V4.0.3, Pixhawk 1. The quad has no feedback from the ESCs to the Pixhawk, so it isn’t(?) the ESCs. I have run the motors at full throttle anchored to my garage vice no problems. I can’t find any settings causing this behaviour and would appreciate some help.
Thanks, Ray

Log File

How does it perform in Acro Mode? You could try incrementing these down:

Although I’ll say I don’t have personal experience trying for very fast forward flight.

Thanks Dave. I have never tried Acro mode. I’ll read up on it and give it a go.

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